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...
I can only give you one well ment advise: DON'T!
There have been hundreds of pilots who have lost their drones flying over water at low altitudes.
As said before the sensors get confused and this effects the ability to maintain a steady flight...