From my own experience, I had only one bad encounter.
When I was in Cozumel, there was some dude that couldn't find his fin on the small (8 customers) boat while we were under way. Evidently, you leave your gear if you dive the next day on the boat, so, it was the "next" day for him. Boy, was he a 3 year old who misplaced his toy... He was openly telling the crew that they forgot his fin and it was their fault. Finally, the fin was found after a while but not before the start of the trip was ruined by a 45 year old who was very confrontational and infantile. I prayed I will not get paired with him. But, as Cozumel usually goes, we dove as a team, not really having a designated buddy.
On one of my liveaboards, there was a lady that was somewhat opinionated. As I found out at the end of the trip, she really was despised by the crew, perhaps, by divers too (though, everyone masked it well). She was ok in my book, I am opinionated too and she reminded me someone from the past, thus, I didn't really notice anything and it kind of really surprised that the crew had such strong negative feelings about her. Made me think what people have thought of me...
However, overall, I am proud that the majority of divers are cheerful and kind people, ready to help out, share and etc. So, my point is, yes, there are some bad apples but if you are a new diver reading this thread, don't despair, it is not how it sounds. People tend to remember bad better than good times, therefore, we dwell upon the negative. You'll find great people diving with you 99% of the time.