Graham.
I have one that I bought for my Bayraider, not just for recovering onto the trailer, but also for assisting the boat/trailer combination up my driveway which is on a slope. However I found I didn't really need to use it after all as the boat is so easy to recover and - after I widened and enlarged my driveway - I can easily just tow it up.
I use it now just for general assistance around my garden and garage now. But I have included the use of it in my list of 'things to consider' for my BR23 when it comes. So I too have an interest in this matter,
It will easily cope with the BR with full tanks, comes complete with a bracket that fits over the tow bar, though not with the trailer attached! But it would be very easy to make up a bracket to fit on the trailer A frame. It is therefore easily and quickly demountable. I haven't dunked it in the sea, but it does get wet/grubby/abused - I have used it for recovering small cruisers on trailers up a muddy slipway without problems.
It has a wired remote control with a decent length of cable. Steel cable(30 ft) with a meaty hook on the end.
However, this model (and I suspect all models) requires a direct connection to the vehicle battery (mine is with crocodile clips). The currents required would I suspect very quickly burn out the 12v 'accessories' socket.
As an aside - I also have one fitted on my 28 foot cruiser which has an electric lifting keel - essentially exactly the same, this one is an American 'strong arm' model (still available) - the keel weighs 700 lbs and is easily raised in a direct lift - again with the wiring direct back to two 110 aph batteries.
The only drawback I have found with them is that they are very slow in operation - which I suppose is no bad thing.
(Just nipped out to the garage - the one I have is a Rolson branded model 12v 25w 300 amps. 6000 lbs rolling pull or 2000 lifting pull - I forget exactly where I got it, but suspect it would have been Maplins).
Hope this is of some help?
Andy