Hi,
Just make sure it is a pure polish, preferably with lots of real carnauba wax, failing that silicone waxes.
Make doubly sure it doesn't have any cutting compounds in the wax - boats are out in the sun all the time and the surface film of GRP or paint tends to degrade - so the cutting compounds wipes off the degraded layer and you have a bright shiny surface. Not a good idea on a car or caravan which have very thin paint layers.
Robert