we were on the july 4th wonder cruise last year....
i think they were planning something special for the sailaway party (on the 4th) but because of lightning, our party was indoors (bummer....totally cramped...no room....we left because we couldn't see a thing)...
however, during the course of the weekend they did lots of other 4th themed stuff....special performances by the characters, special buffet themes, a really great themed sailaway party in nassau, etc.......
i don't recall any fireworks, but we didn't miss them....