When I cruised on the seven-day eastern carribean on the Magic last May, they moved the Pirates night deck party inside onto Deck 3, 4 and 5 in the lobby outside of Lumiere's. I must say, even though it was inside, they did a wonderful job of pulling it off.
Not sure what they do for all (e.g. sail away) deck parties, but I'd guess they would move them inside too.