https://youtube.com/watch?v=asv2bnrBg_c
Lucille Ball claimed that she almost drowned while filming this scene of The Lucy Show.
Speaking of movie accidents related to water that manage to end up in the finished movie:
When they filmed "Das Boot" in 1981, during one scene, they simulated a storm by splashing a 1:1 mockup of the Tower with large amounts of water. One of the actors lost his grip and was swept off the set, leading to one other actor unscriptedly yelling "Man Overboard". The actor that was swept off, Jan Federer, broke a few of his ribs and got a concussion, but they kept the video material and went with it. They had to rewrite the movie, so his character was bedridden - cause the actor actually was. They had to pick him up at the hospital every day when he was to film his scenes and then brought him back. In the uncut version, you can see him lying in his bunk and the strained, painful expression on his face is actually real.
Some more interesting facts: The set they used for the submarine was actually completely closed, there were no cutaways for the camera, which helps to establish the movie's claustrophobic atmosphere.
The actors dubbed their own english dialogue. Furthermore, they were forced to stay as much indoors as possible before they started filming, so they would be as pale as possible, to fit the setting.
In general, the production of the movie/series is rather amazing and if you haven't seen it, I highly recommend it.