Yep. Isn't that funny? But you can put it on a water plane, I expect, and have the real terrain duck where it needs to get deeper.
The good thing about it being terrain is that the reflectivity and waves are not diving under or up from the beach in unwanted areas. And underwater colors can be faked, faster to render as well, no hard calculations, you know. Only if you want 'real transparency', you need something else.