I think the issue is assuming any dog in any embellishment makes someone think it can't possibly be a service animal and in more recent years the go to is "they must be just an emotional support". We all get it, people like to make up stories to have pets with them but not all places are really all that suited for that to be more likely. A cruise ship is a confined vessel, things like bathroom access, food, etc all of that is so limited it really becomes a much larger burden to just make up all of that to have your dog with you.
A dog dressed up most certainly can be a service dog. Do people not think those with service animals do things for Halloween or other such holidays? Or just random days? Like they aren't allowed to do that? That ends up being a problem with other people being presumptuous as if those with service animals need any more of that stuff. A service dog would need to be able to treat any adornment as a non-distractor and not interfere physically with their tasks at hand, other than that that's it.
I don't think the OP meant anything other than just working (no pun intended) with a preconceived notion of what service animals are and aren't.