I like linseed oil outdoors because it penetrates good and you don't have to do much with it if you're not looking for a fancy finish; just wipe off the excess and let it soak in and dry. I've never used polyurethane in this kind of an application, but it's really tough, sheds water good, and might work well. I did my bed in spar varnish but I was looking for a show finish, and it turned out great. Linseed oil would probably be best...you could just re-coat it from time to time.
