I agree with the above poster. My two teens would prefer NOT to dress up, but they realize that it is just part of the whole "cruise" world. They also have a strict dress code at their high school. They don't love it, but realize that it is in place for a reason.
As far as the cruise, just remember that those pictures are and will be very precious to your mom. I bet you looked great in your tux! I teach in an inner city school. Most of my young students will never get to experience a cruise, so give your mom and a hug and tell her that you'd be happy to wear that tux again for your NEXT cruise! : )