I tried it and it was a near disaster. At least using the stock app, as it wanted to autoland all the time because the VPS shuts off over water. It was a constant fight to get the drone back onboard so no photos or video were made. I was happy to have Tello back onboard.
There is two things you can try. First is to put some tape over the downfacing pinhole camera. VPS will be off but short indoor test flights did not trigger the autoland. Or maybe they have updated the firmware to no longer autoland on VPS lost? Need to try a bit more.
Second, if you are a TelloFpv user: I just added an option that cancels any landing if you use the throttle. Maybe I will change that to cancel any autolanding that I can get hold off. That feature is not yet release, will go into the next release.
What seems to work is flying over shallow, totally flat (0.0kn wind) and crystal clear water as Tello seems to use the seabed then. But I bet distance & position calculation will be wrong so RTH won't work well
You should be allright as long as you don't stop to hover. It will not hold position and some have reported that it starts to autoland. I suggest you practice at the beach/lake before taking it out on a boat.