I've seen large groups (family reunions) wearing the same logo... like "The ___ Family Reunion From Minnesota" but each family had their own color. I thought that was neat. The original parents had one color and each of their children had their own family color that the spouse/kids also had. You could also order t-shirts from the cruise line you're sailing on and have the dates/names put on the backs. Call around some printing shops and see if they do t-shirts.
check you local trophy shops, my kids play sports and most of the tropy places here also do shirts. A team order here usally is about $11 a shirt for a colored beefy T Hanse shirts (hold up really well) with a three color screen print on the front and a name and number on the back. Definately shop around!!