Haha, I think Marian wanted to know where DangerMouse was suggesting, not how to do it in game specifically. xD But thats a pretty nice code there anyway.
I'll have a go at simplifying:
Marian, you know the XML file that you had to create to make your map? If you open it in notepad (or any other text editor) then you should find at the top line that looks like this:
<Level tileset="energytemple" waterLevel="0" gradTop="0.6 0.1 0.1" gradBtm="0.1 0 0" gradient="1" parallax="0.5 0.55 0.6" music="miniboss" sceneColor="1 0.7 0.8" />
The bit in red is the part we want to look at, it should be the second parameter from the left. The teal number there, '0', is the number you want to change if you want to have a water level. If it is a number other than 0 you will have a water line. The higher the number, the lower the water level will be.
If you experiment a little you can see where each number makes the water level go, and find the number you want to use.
Say for example you wanted it to be at 4000, you would make the line look something like this:
<Level tileset="energytemple" waterLevel="4000" gradTop="0.6 0.1 0.1" gradBtm="0.1 0 0" gradient="1" parallax="0.5 0.55 0.6" music="miniboss" sceneColor="1 0.7 0.8" />
And then save the XML file. Next time you load your map in the editor it should have a water level somewhere.