Maxbotix rangefinder problem, bad measure in flight

Hello!

We are working on a drone to take water samples for a lake, and I need a very high precision on the altitude.

First we use a laser sensor, but doesnt work very well in the watter.

Now Im trying with a Maxbotix 1204 sonar, and as you can see in the picture, seems to be not very reliable in flight… I think the sound of the props interfere in the measure.

What you will use to have a precision distance with the water? for 7 meters max.

Thanks!