Happens quite often over here in FL. Seen it myself 2-3 times, and almost happened to my truck.
I was in the boat, buddy was backing her in. I wasn't quite deep enough so I said come on back. I launched, and said "Ok - pull forward." He was still in R and hit the gas - water was up to the tailgate. Couple more inches and she would have floated off. With a strong tide that probably would have been all it would have taken.
I also have my shifter cable break on my F150 (technically it was the little plastic piece that shifts the transmission and not the cable) - so whatever gear you happen to be in or going to is where you're stuck. Mine happened to break on the way to D - so N. So...the shifter says D, but you're in N. You give it some gas and unless your left foot is on the brake the boat goes backward. Do that twice and it's going for a drink.
If you panic it's game over.
I used to chuckle at the ineptness of people, now I think twice before going "What an idiot." Then again some of them really are idiots...but at least I think twice about it.
Must have been a slow day for the dive team. Over here it's a call to a tow company.