I got to the Cosmic Cove today, and that's the first time I've ever seen a 3D water world, let alone play one. I always wondered how the controls would work underwater 3D. Takes getting used to. And now, they have an air meter that you have to keep replenishing, which I had never seen before. Makes sense, though.
The water looks so real (except for the falls). And then you can turn it to ice too.
(Don't even know if previous 3D games, 64, Sunshine and the first Galaxy had full underwater worlds).