There's a reason they don't recommend polyurethane for outdoor use. (I
don't know what that reason is, but there is one)
Spar urethane, however, comes in outdoor versions. I've used that on
windowsills in wet locations (over the kitchen sink, for example).
Alternatively, use something that's recommended for decks, although
you might have to recoat it occasionally (at least, the deck stuff
doesn't last too many years on a deck).