Can swim nappies be used in America?

We have booked to go on a cruise and have just received a leaflet a week before going on hols saying that due to the United states Health Policy that swim nappies or swimmers can not be used in the pool I have rang them and they have said that if child is not potty trained then can not go in the pool! How am i suppose to tell a 19 month old he cant go in when his big brother will be in there? I am so mad not to have been told this before when booked the holiday. Has anyone else had this or have anyone else used them and got away with it, i wondered if it was a rule that had to put in small print for law reasons but never actually gets listened to because im sure they would loose alot of bookings if this was the case???