Actually they don't have one on board; but, there was a fellow you had had a root canal before he left and was in severe pain when we were in the infirmary on the Conquest. It seems like they made him an appointment for a dentist in one of the ports (can't remember which one) and he went to the dentist instead of an excursion. So they will try to help you with any problem I think.
My husband was started having an on and off toothache the day we boarded our RCCL ship a few yrs. ago. About l/2 way through the cruise it got pretty bad. The dr. onboard treated him w/an antibiotic and gave him pain medication that got him through the next few days comfortably.