Allow for more water texture quality options
RTT Size is one of those recommended optimizations for water on articles online, OpenMW thankfully has that optional, and in the GUI there are 3 options; High, Medium and Low.
Morrowind and later games also make the texture quality optional too, Morrowind's owm default texture size is set to 128.
I found lower qualities than the ones OpenMW has in the GUI provide more FPS, I made a comparison looking at water reflecting a not so busy scene with the lower texture qualities, refraction is on and reflections are set to world:
Low (512, the currently lowest quality in the GUI):
Lower (256):
Lowest (128):