I agree that a lot of cruise ship people tend to be careless, both underwater and above. Now having said that, my husband and I and some friends who are all avid divers have gone on a few cruises together so I don't count us as the "typical" cruise diver!
Also, we had a horrible experience with some Canadians when diving Cayman Brac and staying at Brac Reef Resort. It was a group from Canada, they were the majority of divers on our boat, and were loud, with their stuff all over the place, and took forever to get off the boat into the water. Disorganized, and didn't care. They also had a handicapped diver who had to be led around by the DM on all dives and they never tipped.
So...I'm sure all Canadians aren't like the above, but they certainly put a sour taste in my mind. I think the better thought here is that discourteous people can exist anywhere. no matter where they are from. In general, I just find that common decency and manners are in shorter supply.