Not sure about all of the lines, but Princess does.
If you are traveling with a service animal, please be aware that Princess requires notice in advance. Entry regulations vary from port to port, and there are some ports that prohibit the landing of animals altogether. Passengers are advised to consult the local authorities at each port of call prior to departure for the necessary documentation. Princess does not provide food for service animals. The choice of dog litter material is limited and varies by ship and location.
Yes they do .. as a matter of fact when I was on the Carnival Legend last April one of the girls from my office was there with her service dog and a group for the visiually impaired and there were over 100 service dogs onboard.
There were two service dogs on my Voyager (RCCL) cruise two years ago. We encountered them several times in the elevators, etc., and their dinner table was right next to ours - would that some of the kids on board were as well behaved!
I believe they set up a special area below decks for the dogs to do their "business" - but, wow, 100!