Toilets on board of ships have some special requirements. Depending on the state of the sea, the discharge will be, let's say "the wrong way" unless special equipment is used. The modern solution is a vacuum system just like they use on airliners. Everything is collected in a waste treatment plant and then discharged overboard or put in a holding tank when in port. The company Jets of Norway makes the most commonly used vacuum system on ships. Vacuum systems have a lot of installation advantages as well AND they save huge amounts of water. Instead of 2-4 gallons per flush (8-16 liters) we'll only need 1/3/of a gallon (1.2 liters) because the vacuum is doing the work, not the water.