And how many flight simulators do you know with real water that have the performance of FS2? Or FS4 ( probably )? I do not know anyone.
Shaders on water does not so significantly impact performances on modern hardware. You can have water that looks amazing from the sky (even if not true 3D water) for a cheap price performance-wise.
The main problem (I assume) is not the shaders themselves but the fact that the engine has to "know" where water is, with a high precision. The limits between water and not-water have to be globally defined, which is not obvious if you use raw aerial photos.