I'll start this post with a disclaimer:
I'm not here to praise the game, because i honestly haven't played enough to have an opinion on whether it's good or not. I certainly do not think it's bad. What i'm going to attempt here is provide honest and constructive criticism.
First off, what i did so far:
I downloaded the demo. Installation was nice, i like how you put the game in a directory i the start menu that's actually called after the game and not the company. More comfortable and shows where your focus lies. I disliked how the installer kept the bar at 10% for 90% of the installation, because it progresses by file, and got stuck on your huge data file. It left me dreading how long such a small install could take, and how much data the uncompressed game will take, as i clicked through the previous screens so fast, that i did not even notice when it said that.
I usually go for "Configure" first, so i did just that. Keep however in mind that many people will start the game first and many people have quite varying setups of differing power. This i will have to complain about your defaults:
Fullscreen: This can take considerable time to enable when cold-starting a game, especially when memory is low. Your game is quite demanding, so please do not make this a default option. The sanity of your users will thank you.
Vsync: I know what it is. Who else does? There needs to be some kind of explanation for it, a tool-tip, as is customary in Win32 user interfaces. Additionally: This is an option you only want to enable on powerful machines, as tearing only occurs massively when you go above the refresh rate of your screen with your fps. Thus, label the tool-tip: "Activate this on fast machines to minimize graphics tearing." Default it to off.
Framebuffer Effects: Explained in the bottom right and has that text in grey. Good, perhaps
bold "Framebuffer Effects" in the notes.
Mipmap: I can guess on what it means, but even i don't know for sure. Explain it somehow.
Resolution: Good choice. Maybe consider selecting the desktop as default resolution, but the one you have is nice in any case.
One thing i disliked: I can't move the configure window. Please never make windows stuck in the center of the screen like that, it's just rude.
Clicked save, clicked dive.
Loading bar, nice. However, it never reached 100%, but went into the game at ~75%. Make it reach 100% somehow, then fade over. It feels nicer to the user.
The line effect on the bitblog logo (i use 640x480x16) just looks lazy and ugly due to distortion. Please make it take note of the resolution and scale the texture so the lines look uniform.
Main screen: Lovely, like the swarm thingy. The texture of the aquaria logo draws an ugly horizontal line through the screen, transparency needs tweaked there. Options and Mod buttons are too dim, i only saw them on the third start of the game. Perhaps make the text glow a bit with plants or something. The green water plant behind the R looks unnatural due to too strong stretching.
Options menu: Barebones, but nothing missing i could point to off the bat, aside from graphics options. I guess you didn't want to go to the trouble of integrating them properly into the engine. Shame for those of us who like to tweak them to match our tastes and hardware, as we're now forced to start/quit the game multiple times.
Personal opinion: The face in the option screen looks down-right ugly. Sunken lips, beady (i think that's the term, not a native speaker) eyes, pale conflexion, but nothing that hints at it being stone. It seems like i'm looking at a drowned person. Could at least have given her a smile. Smiles do a lot make even uglier persons more beautiful.
New Game:
The intro text is rather confusing. I won't go into too much detail here, but DO grab people who've never played the game, plunk them down in front of it, have them watch the intro, then ask them to tell you what they saw and heard. You will find it greatly differs from what you intended. One example: She mentions inhabitants of the cave, yet there are no plants nor animals to see, only dim, small shadow of background fishes.
The intro is also missing something else: There is NO emotion. At all. Does she welcome the player, is she happy about the chance of telling her story? Is she sad about what's going to unfold? She seems emotionally speaking, dead as a fish. If you can't think of anything else, at least make her smile at the player. She is visually not particularly endearing, and that will need to be off-set by personality.
When swimming around, she flips left-right. However that is VERY abrupt. At least add one transition frame to make it more fluid. When standing still, make her slowly straighten her position to up-right, instead of reamining at an impossible angle. Additionally, maybe have her align her angle to a surface, if she's sufficiently close to one. Also have her move her head around to look at whatever the mouse is pointing at, it will deepen the connection between her and the player. The "swim boost" ends too abruptly when holding the left mouse button, have the additional speed fade out slowly. The sprite also seems to skip backwards a bit when the boost ends, that feels quite unnaturally. The spiral animation is also rather choppy and skippy.
Re swimming animation: She seems to flip-flop her legs around rather un-motivatedly and like sticks. It would be nicer if she would move them in a more sensous (?) wave motion, like a dolphin would, or like the people in Seaquest, which i dimly seem to remember.
After swimming around for a while, 5 minutes or so, i found out: I won't be able to play this game. The reason being: My index finger hurt from constantly pressing the left mouse button. This game sorely needs a "travel" function. Maybe have a double-click act as a trigger to have her constantly follow the mouse instead of only when the button is pressed.
In my opinion this is rather important."You have found a new map token." What. I didn't see my sprite touch anything aside from one of these jellyfish. Does that mean jellyfish give me maps? Obviously that's not the case, so perhaps make the text "you've discovered a new part of the map." or something less confusing.
"Hold right to sing." Hey, this is neat. *swims up to the top of the cave to have less visual clutter and makes a few melodies, then goes on exploring*
Hm, small cave with a rock and a plant, nothing to do. *backtracks*
*passes the skull* "Hey another plant, wonder what's up with those. *goes on and leaves that map*
Oh hey, i can stick to walls, but hmm, it seems to only work sometimes. Would be nice if the game gives a direct hint as to when i can stick to a wall.
Middle-mouse to look, neat, pointless, but neat. Also, ow, trying to move while looking leads to finger pain.
"Sing the same color note as the plant[...]" Colour? *notices jellyfish, tries to sing reds* Eh? *notices green plant that was blending with the background* Oooooh! *makes it pop, gathers recipe* Hmm, so these plants drop recipes? Best i backtrack to gather the contents of the previous plants too. I notice the cursor shining when going over the skull, try singing a bit and clicking on it, but nothing happens.[break]
This is really ugly here. Rule 1 of any visual narrative and teaching experience: Show, not tell. In retrospect i was probably supposed to notice how singing makes the first plant pop next to the skull, but as the toom was too big, i didn't and missed the chance to learn something in an intuitive manner. Then i go on and find another plant, but as there are no cues around, i fail to learn what singing does to those.
The second plant would have been the *perfect* chance to tell the player to sing, if he, due to bad level design, managed to not notice how to pop the first one. Instead however, the learning experience leads to tedium, due to backtracking forced by the fear of missing something important ot valuable.
[/break]
I go on, am surprised at the game telling me about the swim boost, as that was literally my first movement in the game. Find the plants. At that point i would've liked to hear a little comment from ... What is her name? Aquaria? I can't remember having been told her name before. But seeing how she seems to be rather 
anyhow, i can't bring myself to actually "care" about her. Anyways, i would've liked to hear a comment from her about how they reinvigorate her or something, instead of the game blandly going: These are health kits, lol.
I take care to explore the map completely, going upwards abov the shooting squids and find another recipe, yay, the first genuine piece of gratification in the demo.
Then i go down and find the red crystal. AHA, save points, they look beautiful. ^_^
Oh, right-clicking on them. Hmm, cursor shines... Wait, what. Ooooh! *goes backtrack to the skull* After all, it might be something important! 
*arrives at the skull, right-clicks it* Ok, she sits down? *waits a bit* Nothing happens... *clicks to get her up, right-clicks again* Well, that was an amazing waste of time, how brilliant! *goes back-track to the red crystal to save* I hope the rest of the game doesn't involve as much backtracking...[break]
Second time you managed to not properly teach. You had a perfect chance with the skull there, could've put it in a room before or after a smaller room that should've contained the first singing plant, and could've told me to right-click it then, perhaps mention that it only lets her relax a bit. Instead the whole thing turned into a massive pointless ordeal that rather turned me off the game.
[/break]
Ok, game is saved, my hand hurts and i've got enough material to write a book on this, let's get outta here for now.
*hits alt+F4, nothing happens*
FGSFDS!
*hits ESC, then quit game, clicks yes, watches black screen and waits for the game to quit, gets assaulted by music and the main menu*
WTF!
*rather annoyed now, hits Exit, yes, waits again at the black screen, then: AD screen*
Ok, what the christ. Did i accidentally download something from Popcap games? I just want out of here!
*clicks Exit, listens to her ... moan?*
Ok, awesome, pity-grab combined with soft-porn...[break]
Quitting the game should not be an ordeal in itself. When i quit the game, i want out, period. Alt+F4 should never be disabled, ever. The ingame system menu should either offer only an option to completely quit the game; or an option to Quit to Main Menu and one to Quit the Game. At least rename the Quit Game button to Quit to Main Menu, so i am prepared for more delays in getting back to other things.
Also, don't throw ads at me when i quit the game. Seriously. I know it is a demo, i downloaded it rom a link labeled demo, and the installer itself says demo, even the start menu entry does, and i think i saw it ingame too. I repeat, i must be blind and dumb to not notice this was a demo and that i'd need to buy the full game to get more.
However: The screen wasn't that badly designed. Do yourself a favor and make it pop up after going through the content of the demo. I may want to buy it *then* and would appreciate a click link to the purchase location.
[/break]
Conclusion:
I hope the above stuff is useful to you and will deliver one last admonishment: You look at the game with the eyes of people who *know* how it works. Everything makes sense to you and there is no question left. Which makes you the worst people on earth to decide on how to structure a tutorial.
Take hints from Introversion, read their blogs, learn how they use input of people who've never before touched their game to give it a *sensible* interface and learning process that can be picked up intuitively.
