I would venture the guess (based on other voxel games I've played) that it's because the ramp block, while a wedge shape, still takes up the full blocks it travels through. That air volume that fills the rest of the block would still be air if the ramp was placed underwater. They might have it disabled until (if) they make the system able to turn them to water when placed underwater. Just a guess.
Posts made by Arisilde
RE: Ramps underwater...