One of the local groups (100 people) that I have sailed with creates a design contest. They have a pre-cruise Meet & Greet party (in our area) about 2 months before sailing and whom ever wishes to submit a design does so on paper. Everyone votes and the best design wins. This is screenprinted on the front while, everyone also signs/writes their name on a large white poster board with a medium point felt tip pen. All Signatures are imposed on the back of each shirt. The ship name, date of the cruise and Group Name is usually included on the shirt as well. You should be able to find a local screenprinter or Novelty Incentive shop that will do the work for under $20 a shirt depending on quantity. The more that is ordered, the lower the price as well as the quality of the t-shirt. Hanes Beefy T 50/50 cotton, is a good quality shirt.
Have fun with it!