I have never had an op cancel at the last minute due to not having enough divers. If I were paying full price,with a limited number of days to dive on the trip, and an op cancelled at the last minute.....I would not be happy at all. IMO that would be a very poor business practice. Now if they were able to get me on another boat that day, that would greatly mitigate the damage.
You are very lucky. I used to go to South Florida for one week vacations each year, and having dives canceled like that was always infuriating, because that might be 20-40% of my planned diving for that vacation. I did one week vacations for about 10 years, then I retired and spent at least a month for about 5 years. My guess is that over that time, I have had 20-25 dives canceled at the last minute because they did not have enough divers. It does not happen as much any more because I have gotten a feel for a dive that is a potential cancellation, and I don't schedule on those days in the first place.