Monnie, I had the same question about the Carnival Miracle when I sailed in September. If RC is similar, they usually only have a priest on board for major holidays (i.e. Easter). It is possible, however, that a priest will be traveling on your cruise. If this is the case, I would assume that he would celebrate mass. Even if you call RC and and they tell you "no", check with the information desk while on-board when Sunday approaches.
As a last resort, you can spend some nice quiet time in the chapel on Sunday. It's not quite the same as making it to mass, but it's the best you can do and it is usually peaceful there.
RCCL - Adventure OTS (Honeymoon!)
CCL - Carnival Miracle
RCCL - Enchantment OTS