I actually emailed Oracal about using 651 on a boat (for reg numbers) and they didn't specifically advise against doing it. They recommended rinsing it if it got salt water or fuel all over it, and not to wax it but use silicone polish, since wax would dry out the vinyl prematurely (no matter what kind it is).
I would use 651 for reg numbers, but if you are doing a nice stylish scripted boat name (also boat names tend to be lower down on the hull) I'd use 751. Whenever I get around to ordering another batch of vinyl I'll probably get a small roll of 751 in black and white just for those "questionable environment" jobs. I don't really see any reason to laminate. Maybe edge seal?