If the obstructions are wood, you can bore though them to make straight conduit runs.

Your other option is to completely mount the thing under the deck and just run a longer string of lights.

Get a different in-use cover. I've got 6 of these going around the porch, all coming up from underneath and mounted on the posts.

Here is a link that might be useful: Taymac Low Profile Expandable In-Use Cover

Hey Mike. There's outdoor wire you can buy from lowes, home depot, or any other hardware store. Be sure to hook the new wire to the load side of the GFCI and you wont have to worry about the water. As long as the wiring is safe there's no reason you both can't go nuts and do whatever you'd like back there. Look at this article, they went ham!

Here is a link that might be useful: Del mar - Festive Outdoor Lighting

I believe his comments about being in the land of metal conduits means that he is somewhere like Chicagoland that precludes the use of unprotected cable.