I think you'll find that most (if not all) polyurethane manufacturers will NOT recommend use on any kind of boat (maybe interior use) unless they market their product(s) specifically for marine applications, and even then there are usually a LOT of disclaimers.

Wait until you hear the answers about use on a SOF!

: I emailed Minwax with my earlier question about whether I could re-coat
: without sanding between coats (Helmsman spar poly). Here's their reply:
: Thanks for taking the time to contact Minwax. No, we don't recommend
: applying the Helmsman Spar Urethane without sanding in between coats. We
: also don't recommend this product for submersion in water. If the kayak is
: only decorative, then it may work, but if you are actively using it, don't
: use this product over the epoxy coated surface.

: I find this interesting, since I've used the boat for 2 seasons now with the
: spray on version of this poly, with no problems whatsoever, and I know
: others on this board have had longer term success with the brush on
: version. I applaude Minwax for being forthright, but it seems they are
: being a little on the conservative side. On the other hand, my boat is
: only used in fresh water, and it's stored inside when not in use, so it
: has an easier life than other's boats.