If you read the ADA rules .... they do cover instances when a dog which is presented as a service dog may be removed..... those include aggressive behavior and other infractions. The dog also has to be clean (brushed, bathed, etc) I work in dog training and have helped with the initial training of several service dogs who then went on to perform very specific tasks like detection of seizures before they occur. Many of the dogs I trained did not attain the service dog designation even though they were great on the down, sit, stay, eliminate routines. Cruise lines can not ask you what your disability is.... but they can ask what the dog has been trained to do. I think most are just afraid to ask.
I have seen plenty of fake service dogs.... including one on a plane flight that insisted on seating in my window seat (I got the stewardess to move me.... and I love dogs but....!!!