I don't know much a about music, but I am sure you could ask the cruise director to be introduced to the Band Leader. Having met one, they are very professional. If he agreed to let you jam or join in, I have no doubt they would have the appropriate guitar for you to borrow. I once watched the "kids" practice for their part in the talent show, and a young teenager was doing quite well on a beautiful accoustic guitar. We usually prefer late dining and the talent show seems always to run before 8:30 or during our seating, so we have never gone to see a show.
Go for It!