This happened with a group of 4 (including me) shortly after the initial release on Steam. I said "did something bump the boat"? Someone else was like "what the hell is that?". Then we were terrified out of our minds.

Seeing this all unfold I hit the f12 key. We all died.

https://steamcommunity.com/sharedfiles/filedetails/?id=2676374222

Back then we didn't know that inventory on player and boat floated. So we'd always change into our "sea blues" (troll hide) and stow weapons before sailing.