Anything outside is a wet location so you expect some water in the box, in spite of all of those rain tight covers. We do require weather resistant devices don't we?
The trick is to be sure it doesn't accumulate enough water to trip the GFCI.
The code would still require an "in use" cover so that makes this a tad harder. Perhaps a trap door in the deck with a normally oriented receptacle under it would be the way to go.
Then you could just mount a bell box on a joist and you might even be able to get this called "damp" with the right shielding under the deck so you could use a snap cover.
