I think I might be the only one, but I'd really like for the water world to feature a "not-so calm" part. When people think water world, they think beaches, beautiful sunsets, exploring underwater in calm environment... Don't get me wrong, I love these settings but I think there's more to it when doing a level around the sea.
What I'd really like to see is a part of the level being set in a raging, stormy sea with thunder, lightning, rain. Imaging climbing a cliff being beaten by the sea (you could have to hide behind rocks when the water's coming like in DKCR), bursts of winds would try to push you off the platforms you stand on or are trying to reach, etc.
As for the underwater parts, maybe add whirlpools, current, etc.
Anyway, that's just an idea, what do you think?