I've not used it on a boat, but on the cars I use a Porter Cable 7424. It's a variable speed random orbit polisher. It doesn't spin which is advantageous for the car as it won't "burn" the paint but still does an excellent job at polishing. I'll then wax it with the same machine (different pad).
I don't know what the risk is on a boat for "burning", however, like Bryce said, angle grinders do spin huge RPM's. If you want to go the spinning route, go with a "variable speed" polisher like a Dewalt. I've used those in the past with a medium cut or a fine cut compound and it does wonders. Again, this was on paint. Follow that up with polish, then wax (not using the spinning polisher, just get a Fleet Farm cheapo waxing machine).