In order of increasing durability: conventional, 1-pack poly, 2-pack poly.
2-pack poly is the dog's whatsits. I used it on a GRP kitcar 15 years ago and it's still perfect. BUT - it mustn't be allowed to get damp until it's fully cured, i.e. 48 hours. I had dew form on some the day after painting and it went chalky after a few months.
Don't apply poly on top of conventional but probably safe the other way round.
just a thought - I painted a tender red and ended up getting colour on the boat hull from the tender whilste on a lively anchorage - IMHO paint the tender the same colour as the boat hull and avoud this problem
If you want to avoid it being nicked, paint it with Woolworth's enamel, laid on nice & thick so there are lots of runs, then give it a thin second coat of a contrasting colour.