Holding altitude above water surface

I am trying to determine the downward distance between the drone and the sea level. I have onboard an IR rangefinder TeraOne by TeraBee, but of course the IR beam is scattered on the water surface, the barometer is not accurate with its AMSL reporting.

I have only seen a radar sensor that claims to be able to measure above water, but its too big and too expensive for just one job. Is there any other sensor I should try ? will Ultrasonic sensor work above water surface ? Stereo vision ?