I know some of the newer searays have dark colored hulls so it's quite possible that it is gc. If it was an older searay i would say that it had been painted. The white specs are dried up salt,...
Most custom sport fish will be painted, as well as all the bigger yachts (100+ ft.) The more you do the easier it will be for you to tell the difference.
Did it years ago on an old hatteras to see how it would look. It didn't really work, but the varnish was in really bad shape. I suppose it may work if varnish isn't too far gone.
If you have compounded, and the surface is clean (no oxidation, or hazy areas).
I would polish with a yellow wool pad, then use 3m's ultra machine polish with a foam waffle pad.