Just keep your eyes open. Use your gaydar. Be open and friendly. Look around the deck on sail-away to see who is with whom. Couples are easier to spot, but just strike up a conversation with anyone you see that is interesting. Everyone on the ship is there to have a good time, is people are usually outgoing.
Try the disco late at night, the gym during the day, and the lounges before dinner. Go with an attitude of meeting interesting people, and be interesting yourself.