I've used Minwax Polyurethane on two different boats for over 16 years. I did three coats to start with and every few years depending on wear I'll freshen it up with a new coat, just sand it with 220 and apply a new coat.
My first boat had 3/4" thick teak and holly floors so they were not removable, for the Catalina I did seal both sides for moisture protection, so far we're at two years and going strong on our 34.