If you read article 300.4 I beleive you would be fine with UF under the deck. #1 the UF is rated for exposure and #2 the deck with limited access affords the required protection. Even if the deck was exposed under, as long as the wire is bored through the joist or stapled to the side of the joist, you meet 300.4. This is my opinion and if I'm wrong someone will correct me.
