At least if the LiPo is empty the Tello will land. Doesn't matter if above water or not. But this could be the last time for the Tello.
I saw some YouTube videos where the Tello was flying over water and it didn't look really secure. I wouldn't try that. That's why I have no experience with flying over water and I think I will never get it.
It's very dangerous for tello, tried to fly over water 30m high and 100-150m far, everything was fine until I released sticks and (maybe) drone saw that VPS is unviable. Anyway, it was the only time when tello tried to autoland(!) with >50% battery. I was scared, but did cancel autolanding and brought him back. So, if you really want to try it- only in windless conditions and not too low. Also, don't fly above big, noticeable from above waves, tello tries to follow them.
Me and my friend were once flying our Tellos over a lake. We were ~15m high and ~5m from shore, when suddenly my friend's UAV started to lose altitude fast and splashed into water. Funny thing is that after a few seconds, when all hope was gone, the engines started by themselves and the Tello took off from underneeth water. I landed mine and helped him take his back on the ground. It's been half a year ago, and the Tello still flies as if nothing had happened. Anyway, don't fly over water. Due to bad VPS data, aircraft can suddenly start autoland procedure and if you don't act fast, you can share my friend's experience.