I did this as a VRT way back in 1980. There were about a dozen of us, given a day's instruction then dumped off a crash boat about five miles off the coast. We were given a multi-seat dinghy to play with, and told that, if the chopper went u/s the boat would come back for us.
Despite it being November, the Channel was a veritable mill pond. After about an hour, we heard the boat coming back, and assumed it was for the pick up. Having opened both doors we were sitting in breathless anticipation when the launch went past, doing warp snot.....
It seems that the trusty crew thought we'd had it too easy and wanted to add a little realism, and three feet of water, to the dinghy. An hour after that, the Sea King arrived. I have to say that, as we went up the wire, it's been the only time in my life I was glad to wrap my arms around another bloke.....
I still have the medical certificate I had to get before hand. I should have known something was afoot when I read that I would ' suffer no ill effects if thrown into sea water under RAF supervised conditions'.....