The only issue is that the downward sensors may have trouble maintaining horizontal position if you fly low enough to the water.
But maintaining horizontal position is not real concern unless you are close to obstacles.
I would argue that for some extend if you won't mind.
Please consider this environment: say the pilot is on the bank or the shore, and the drone is flying on the sea or lake with no other seacraft in sight. The height of the tides, the splash...