Just stop the boat!
Chances are the MOB will have swum up to you before you've opened the swim platform.
I can imagine myself having finally gotten around to where the victim fell in, only to hear the click of a beer can being opened behind me, the mob having already chased me down, climbed on board, and gotten into the cooler for a refreshment.
"Dude, let me know when you've rescued me. I'll be over here relaxing."