While I don't know any of the dive OPs in Cozumel, I do know a thing or two about the services business and if you accept a reservation you just made a commitment to a customer. Sorry but not having enough people show up to cover costs is just the risk of running a services business. I have to agree with Aldora's policy if only one shows up then you take that person out. I would be pretty disappointed if after spending a couple of grand on airfare and hotel someone who I made an agreement with for a service decided to change my plans. No matter how much effort they spent trying to set me up with someone else. I picked that OP based on a set of criteria that was important to me, if I would have wanted a different OP then I would have chose them in the first place right?
This is just my opinion and at the end of the day we are all limited by what we are willing to tolerate as customers.