It is not only about how the animals are killed also about how they are raised (free range vs cages etc). But there are def different circumstance how the animals are killed, look at the pictures from the slaughter houses they have just closed in Belgium, the animals were being abused before being killed, totally unnecessary. Some religious ways of slaughtering also forbid the animals to get anethesia. So there is for sure a difference in the way animals are slaughtered and I think everyone wishes it to be as painless and quick as possible. But I guess it will be difficult to find out where the meat is coming from on a cruise, thereby I also think that not many people ask about it so companies get away with serving the cheaper meat and after all they want to make money.
I saw in a blog that you could indicate if you wanted to eat vegetarian or vegan but haven't seen it when I did the online check-in.