I have been flying continuously over water, and close to it,
so I read about it before doing so. I read it is not good, and I have to agree with the reasoning.
The downward vision sensors are sonar, they pass right thru water, and perceive the land to be the bottom of the lake in shallow clear water or rivers. Also the transparency of water and reflection or motion of the surface causes positioning issues over the ocean or moving lake water. There are also videos of the Mavictaking control and just landing in the water when downward sensors are on, the drone thinks you are within landing range so landing protocol starts without your control.
I don't want that to happen so I never use sensors over water. Just remember to turn them back ON prior to landing near you,
so that you don't slam your Mavic into the ground. You can also hand land, or land VERY slowly, keeping in mind that tilting the quad near the ground creates undesirable air turbulence, and the props can hit the ground much quicker than you expect. I advise tripod mode if you are not really sensitive on the sticks yet.
Regards.