Will the water go on forever, or is there some secret eventually?
Solution 1:
It just goes on forever (like, for example, the infinite climb room). Nothing special about that either, you just have the ability to keep going.
Solution 2:
As far as anyone has reported, the only room with benefits from swimming out to the side of the screen is the one with the skull artifact (after the mausoleum). It was worth a try.