True and I know from firsthand.
My friends' daughter has one because she chokes easily- she is a childhood cancer survior- but the cancer was in her throat-that is a side effect of her surgery that has worsened over the years and the dog is trained to go and warn my friend. Whenever she takes her out the dog goes with them-although since the daughter is 35 now and in bad physical shape-she does not take her out much anymore. As the daughter has aged over the years, side effects of the surgery has destroyed her health-she has to have a feeding tube (the choking is so bad) and she uses a wheelchair most of the time because she is so weak. If my friend chose to take her daughter on a cruise I would not begrudge her having her dog one bit-and yes you may would see my friend's husband walking the dog or taking the dog to "potty." He takes care of that for his daughter.
That is what I would think if a saw a person walking a dog on ship-that they have an ill family member that it is a service dog for the family member.