Well, I ended up managing to slip past one of those fish completely by accident, somewhat to my surprise.

Found my way into someplace that was completely dark, although it didn't seem too close to the other dark place I'd wandered into from somewhere in the Open Waters, so maybe there's two different dark areas?
Anyway, I think now I've been through most of the Kelp Forest, or at least I've found my way to the nature goddess, who has been reliably killing me every time I go up against her. Although that's partly because it took me a while to figure out that just singing the notes she was making wasn't going to be enough... But really, even now that I've gotten the general idea of what I'm supposed to be doing (trying to avoid spoilers here), and can injure her to the point where she goes ballistic and throws everything at you at once, I never seem to be able to survive that final onslaught.
A couple of you said going right might have been easier than going to the Kelp Forest - does that mean Mithalas? Because I did find my way there at one point. I'd been in the Kelp Forest briefly, and then somehow ended up outside of it by accident and couldn't find my way back in at first, so I started exploring elsewhere, and found the entrance to Mithalas... But I'd heard that was supposed to be harder than the Kelp Forest, so I backed out again and kept wandering until I eventually found my way back to the Kelp Forest (the world map seems to be of limited use in that respect, or maybe I'm just not good at reading it).
Anyway, I'm just wondering if maybe I was wrong and Mithalas would have been easier, or if maybe there's somewhere else I should have gone before either of those? Because I seem to be out my league with the nature goddess...