Tie a rope onto the engine and have a helper keeping some of the weight on the rope - then WHEN it slips, hopefully they can keep the engine out of the water.
Dropping the engine into the water is not the 'end' but it is a bit of a job getting it back into a usable condition.
When fitting the OB to my tender it slipped and fell off into about 30 feet of water.
Hours of throwing and recovering a grapnel eventually bought it back - it was nowhere near where it went into the water - I think it must have 'fluttered' as it went down.
It is then the fun starts :
Drain all the oil out and flush with fresh oil, refill.
Drain all of the fuel out and flush with fresh petrol, refill.
Remove and strip carb(s) reassemble and refit.
Wash down the exterior with fresh clean water.
Dry everything off.
Once all reassembled it should start - leave it running for an hour to get hot and get everything dry and flushed thru.
EXCEPT when it doesn't start - I pulled on the starter until my arms ached, choke in, choke out, throttle moved to different locations - nothing.
Stripped the carb again and found a "bubble" (micro globule) of water blocking the jet - blew it out, reassembled and it started 1st pull.