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.