Trying to help out a friend here... he has a small ski boat (not even sure of the engine). He shut off the engine while floating and the engine starting dieseling. He flooded it to get it to shut down. Later, he tried to start it and nothing. He removed the spark plugs and one cylinder was full of water. He cleared it out and the boat started and ran fine.

When he got home he did a compression test and it checked out fine and the boat runs just fine. Did he possibly suck some water through the exhaust when dieseling? Sounds like he might be lucky he didn't bend some valves.

Any help would be appreciated. Thanks.

Yes

Yes, likely reversed rotation, Common result. Better determine why it dieseled. It will happen again

What is dieseled? Noob question, lol

When a gas engine continues to run after the ignition (spark) is turned off, like a diesel which does not have spark plugs but instead uses heat and pressure to cause combustion.

Sounds like a diesel too when it's happening . Too high idle and improper ignition timing will cause it among a couple other things .