Also I would advise you to shut of the Visual Positioning Sensors. (VPS)
They can get confused by the surface of the water and give false information to the brain of the drone about the altitude.
Especially when flying over water in a wind still...
A safe altitude is when you can see the reflections of the water don't shimmer on the drones surface.
A minimum of 10 meters or 30 feet is my safe lowest flight level, but on a very sunny day with more reflection on the waters surface I will fly...