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...
Staying a bit high over water is your friend. The movement of the water can sometimes confuse the sensors and you may get inaccurate readings for altitude from the controller.
If you want to fly really low just be prepared. You may have to save...
It was less of thinking and more of a "hold my beer" process.
A lot of people who do this are less informed about what could happen. If you are a member of this site or anything like it, you are more informed than the guy who bought a drone on...