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...
If you "fancy" a holiday in the US there are millions of reasons to still come. You will NEVER experience scenery like the Grand Canyon, the national parks of Utah(Arches, Canyonlands, etc) , the dark skies of the American West for...