What could I possibly be building in here? You'll find out soon enough.
This was an absolute pain to build. First I had to build the rough outline, build it down to the right depth, fill in the walls, level the bottom, then fill the whole thing with wool (took the longest out of everything else) then light the wool on fire to get rid of it and the water, all while dealing with the slowed movement speed you get when you're in water. The result you see above. Hopefully, it'll be worth (I already think it is; I've finished building what's in there, now I'm just filling it with water and dismantling the dock. Expect pics 'soon'). The 'tower' made of iron is a lightning rod, because... actually, just because.
This was all done in Creative, as I don't have the patience to build something like this in Survival.