welcome aboard....read post 1 for all the fun info about the trip.....
but in answer to your question, yes they do more shows on the sea days. When we did the EBTA in 2007 there were two additional stage shows (one of which I really loved and one of which was kinda painful
), several additional cabaret acts (puppeteer/ventriloquist, musicians, magicians, etc.) and a couple celebrity performances (on our trip it was Boys II Men and Jim Brickman). They also sometimes use the big theater to show a major Disney release film----on the EBTA I'm pretty sure they showed the premier of the POTC film that was released around that time (and on our recent 4 night on the Wonder, even, they showed "Earth" as one of the evening entertainment/show options).
DCL definitely doesn't leave you bored in the evenings!