You never fly over water?
It's far safer than over land.
No obstacles to run into and a completely uninterrupted signal and sight.
Only thing that would ever really worry me is crossing into a GEO zone over water that might cause a hover that could lead to an autoland, if the app crashed, and you couldn't reauthorize your permission to fly there in time.
As I said never felt the need to risk it. Since DJI came into being they have pretty much either said that is a No-No or highly discourage such flights. It is with that and the fact that my investments which are over $7000 for flying DJI products are more important than the thrill of skimming water surfaces. DJI recommends such flights for advanced pilots and professional aerial photographers and videographers.
All too often you will see the newer fliers just "Go for it" and end in tragedy. The fact that VPS is recommended to be off (When it is in play) and the fact that temps and with that barometer reading will vary greatly over water surfaces just not a good practice IMHO to be doing the skimming shots. All other water shoots for me can be accomplished on shore and adjusting gimbal angle and using my zoom function to simulate a water fly over.
But yes I have over flown small bodies of water when traversing on a shoot (Swimming pools and small streams, some waterfall shoots) I have no obstacles when flying over land (well 99% of the time) as I accurately plan my flight routes and survey areas of interest long before I "Film". I also try and maintain a VLOS (As per radio signal) so for me that isn't a reason to do it either.
As for GEO, I also know my areas and know where I can fly with DJI. That being said if a authorization was needed to fly a area, I wouldn't worry about a App crash as I have never had one. I am also unsure that once authorized for a area if that isn't already cleared to fly regardless of App after, from reports I have seen it takes a power down/reset of aircraft to clear a authorized spot.
Once flying the aircraft GPS will not hover my aircraft as it uses that to detect a NFZ and wont let you enter a area not authorized, so I wouldn't have to worry about crossing into anything for several reasons, those being the safeguards that DJI employees and common flight training. If at anytime I would lose actual sight of my aircraft or down feed I would just press RTH. I know I haven't crossed nor fly in a unauthorized area so RTH is going to work to bring me home safe.
I will leave the intercepting cruise ships, flights for distance bragging and other such water endeavors for others. That is their choice to make and again if they feel the need to take that water shot while actually over water that make recovery impossible that is on them... As I said to each their own.