This is not a real answer to your question but . . .
While we were on the Sun in October one of the staff members (I've forgotten his position but I think it was one of the asst. cruise directors) led a non-denominational service. Granted, he is not a priest but there was a very nice protestant service conducted. The Sun has a nice little chapel which hold about 24 people but this was held in the theatre. You could probably call NCL and see if they know. If they can't make mass they could at least make a visitation to the blessed sacrament while in any port. There is a very nice Roman church in Cozumel and I'm sure in Belize, although there I usually visit St. John's Episcopal Church since my uncle was vicar there many years ago.