I have used a heat gun, very carefully by the way. Just enough to weaken the glue. It really depends on how brittle the decals are and they glue they used. Sometimes the glue will come off with the sticker, other times it stays on the fiberglass.
I use the erasure wheel in my 14v cordless drill. If you haven't used one you don,t know what your missing. Truly a case of the rite tool for the job. If you get one you will never use ahead gun again.
^Hairdryer and razor blade. Get the tape just warm enough until the blade slides under the tape. Different colors and different styles will differ in how hard it is to get off. Any residual glue, I clean off with acetone. After removal, more than likely there will be a slight color difference in the gel coat. You can wet sand this area and blend into surrounding gel coat. I used 600 grit and then 1000 wet. Then compound and wax.
The yamaha wave runners are smc with an epoxy paint. The erasrers were developed for use on auto paint so no problem. Yacht doctor you really should give them a try no skill required to use one no risk of scratching the paint like a razor and faster.