I wouldn't think that it's any different doing a boat than doing a car in regards to keeping it stuck? my guess would be that your tint guy doesn't want to warranty the stuff since it probably doesn't say in the brochures from 3m that it's designed for boat use in the water all the time. the back of the boat should be relatively cheap since it's a pretty flat surface and I don't think i'd run it around the corners to the sides as that's where you would potentially get "lift" from water forcing itself under the leading edge. I don't know that I'd personally mess with the swim platform as that's a small area and would probably lend itself to peeling.

I had 3m clearbra on my last evo. that stuff is amazing for protecting rock chips. sold the car after 1.5 years without a single blemish on the front end. next new car will have it again. def. worth the cash imo..