I'm okay with the true aquatics waiting until a later MM/DMG to show up -- they're rarely used, and hard to make miniatures for.
Amphibious creatures, like sahaugin, kuo-toa, chuul, and aboleth are a different story, and I hope we see plenty of them.
I still really want to see a Kraken of some kind, but I'm not sure exactly how best to make it work -- perhaps individual tentacles on Medium or Large bases? You could also have a "maw" that the tentacles pull the target over to to devour them.
In fact, I bet a Kraken would be best handled with the trap rules, come to think of it...