We found the cash-only thing to be strange too. It seemed like a lot of money to be carrying in your pocket. You don't have to pay until you arrive so if you cancel at the last minute, you don't have to pay anything, unlike many other hotels that you prepay with a credit card from a booking site well in advance. With those, there's often a penalty or no refund if you cancel within a couple days of the visit. We took out USD cash at home before we left so there was no hassle with finding a safe ATM late at night when we arrived. As far as Aldora Divers, they are the only operation I dove with so I can't compare with the others. It is pretty much the most expensive operation in town as far as I could tell. Their selling point is that they use larger tanks than the other ops so you get longer dives. For me, the main selling point was the convenience of being picked up in the morning by the boat literally a stone's throw from my room. I could have chosen a cheaper dive op, but saving $20 for me wasn't worth it to deal with early morning taxis or whatever to get to the docks in town to meet up with all the other dive boats. Keep in mind, the published dive trip prices are just the base price and then there are all the add-ons like taxes, marine park fee, gear rentals, etc. Nitrox cost an extra $20 per tank and for many of these long dives at an average of 60' depths, Nitrox wasn't optional. My 2-tank trip ended up costing close to $300 Canadian (over $200 US) plus a tip. That's almost double what we pay for local dive charters here in Canada. As far as the hotel Villa Aldora goes, we booked it for a few reasons:
1: The snorkeling. My wife doesn't dive, but we like to spend much of the day in the water so the convenient snorkeling here was the main reason. We didn't have to worry about driving to a beach, finding parking or figuring out where we were going to stash our wallets, keys, clothes, etc. when we were in the water.
2: The location. It's just outside of town so it's quiet with no loud thumping music from nearby bars or beach clubs, but it's close enough that you can walk or bike into town. The hotel has a bunch of bikes that you can use for free. There is a strip of a few waterfront restaurants between the hotel and downtown for an ever quicker walk or bike ride. I've heard a negative is that it's North of town, while the diving is South of town so the distance makes for longer boat rides to the reefs. I personally didn't feel like the boat rides were long or anything so that wasn't an issue for me.
As a final observation, the staff at the Villa Aldora were helpful and friendly. They have a kind of social meet-and-greet for the guests about once a week where they put on cake and snacks and beer/drinks. We're pretty introverted so we didn't go, but I guess it's a good way for normal people to socialize with each other. Our unit (Crystal Suite) didn't have a full kitchen, but it had a fridge/microwave/coffee maker and a drinking water dispenser. We were pretty disappointed by many of the restaurants in town. Maybe we're spoiled by good Mexican restaurants up here in Canada, where they have lots of fresh, spicy ingredients. Almost all the restaurants in Cozumel we went to, including a few local places well away from downtown, were very plain and bland. The tacos were usually frozen meat, chopped up and cooked in a microwave and then put in tortillas with no sauce or salsa or vegetables. We thought it must have been a mistake, until we went to other places and they were mostly all like that. We ended up going to one of the big supermarkets and getting various spicy dishes (mostly mole) from their deli section, some fresh salsa and a stack of tortillas and making our own at the hotel. Having that on the balcony of the hotel with some beers a few feet from the water watching the sunset beats any crap restaurant.