While EMT training can get you employment in a wide array of employment sectors, cruise ship lines are not one of them. That is not to suggest if you take a cruise you might be left without medical aid or emergency medical services. Cruise lines usually employ nurses and doctors. There is little need to employ an EMT or EMT paramedic as most of the ships are fully outfitted to deal with the majority of medical emergency or situation. Remember, EMT training and paramedic training supply a profession usually reserved for "first responders", offering essential emergency care services when medical facilities are not often accessible.
Each cruise is very different too, not only in how the cruise line is run, but because numerous ships are flagged out of other countries. Many countries have no idea the quantity of EMT training and EMT paramedic training that go in to becoming certified as an EMT or paramedic. Many countries also have a totally different definition and knowledge of the medical training and medical ability of those trained for emergency services. I personally had an uncle who had a diabetic incident while on board a cruise. His understanding of emergency medical care and that of the doctor on board were every different. There was no "first responder" EMT or paramedic available. However, once his condition was understood by a staff member of the cruise, he was immediately put into the ship's medical facility and given adequate care. The only real difference was there was no EMT trained personnel who responded promptly to see him safely to the medical facility where he'd be cared for. That was left to his wife, my aunt, and the staff.
This is not to say that a person with EMT training does not have any chance to operate on a cruise line, should that be your desire. Many cruise lines hire contract work, employing qualified EMTs and EMT paramedics. Their role is to come aboard and instruct techniques such as ACLS (advanced cardiovascular life support), PALS (pediatric advanced life support), and BLS (basic life support) to the medical staff, and re-train or re-certify a number of The staff and crew in CPR (cardiopulmonary resuscitation) and the utilization of an AED (automated external defibrillator).
Nonetheless, most cruise lines want and employ nurses or doctors. There are a large amount of organizations hiring those with EMT training and EMT paramedic training, and you should consider looking elsewhere. If you wish to go on a cruise and were seeking to work your way on board, bypass that idea, pay the full price, and go enjoy the cruise.
One last note: Occidentally cruise lines will make special offers, including discount rates on specific cruises, to a unique selection of prospective customers. Often these offers are directed to customers who are active, retired, or volunteer EMTs or EMT paramedics. All you need to do is present a copy of your EMT training certification or EMT ID badge and a driver's license when booking and you will be eligible for exclusive discounted rates.