We were aboard the Sun Princess last year & the guy in our adjoining cabin had his ukulele with him & practiced every afternoon before lunch. I would return to our room & enjoy the music while I waited to join my wife for lunch. It wasn't all that loud but I suppose any musical instrument could keep someone awake late at night.
What even the outcome with the cruise line about taking along your guitar don't miss out on the opportunity to go on the cruise with your dad. You'll look back some day & realize it was one of the most memorable times of you life.