I hope building over water is a feature in 1.0
It seems to strange to me that this isn't already in the game, if you can reach the bottom with your cursor than you should just be able to place structures there, I can't even think of a reason why that shouldn't work. I mean you can actually snap pillars to the bottom and use them as support for ceilings for crying out loud, it's already possible, but then when they're down there you can't remove them because they're in a spot that you aren't supposed to be able to reach, even though they're sitting right there!
I want to see more aquatic content in general, like deep sea fishing spots and open ocean alpha pals, possibly even the ability to build you own oil rigs, but just building on the water would be perfectly fine. They wouldn't even be adding a feature, they would be removing the artificial limitation that won't let you do it (which is another things that drives me insane, stop putting hard caps on my creativity for no reason)