I think in "most" divers are those who are certified and dive only while on vacation, that might do one or two trips a year and rent most of their gear. They spend 10 days in Hawaii, the Caribbean, or other warm water destination with maybe only half of those days diving. They are not necessarily active divers: those that are wet at least once a month, those that are planning their next dive outing while hanging at a safety stop, those who own their kit because renting it for each outing would be astronomically more expense then purchasing it outright, those who educate themselves on various aspects of diving where even if they don't participate in it, they want to at least having a passing conversational knowledge into the subject because they spend time socializing with other active divers.
For the vacationing/occasional diver the price of the app, at $10/month, is less expensive than purchasing most any low end dive computer for the amount of diving they do. They want to have a recorded log of their dive they can take with them, but their normal DC sits in the middle of the console of the rented regulator set they last used. They happen to have the Apple watch because it was the latest and greatest version of wearable tech available when they went to upgrade their existing digital watch. Perhaps they also looked at it from the standpoint of being able to use it as a DC on their next annual or semi-annual tropical vacation where they might make a few dives. They can easily rationalize financing the watch through their cell carrier and the $10/month for the DC functionality is less than they will spend for the $10/day for international access while they are on their vacation.
For active divers, the $80/year price tag is steep for what appears to be a very basic DC considering the longevity of a DC. My old Suunto Spyder from 1999 still (with regular battery changes) functions fine. Granted that device then was a fair bit more expensive than the simple DCs today, and if adjusted for inflation would probably be somewhere in the high end DC price range.
Hopefully they don't fall off their user's wrists as readily when in water as the previous Apple watches would. Although it looks like the dive band Apple sells is "only" $99 if you didn't opt for it during the initial purchase and now need something more secure. Fortunately it looks like the aftermarket has a $20 NATO style strap for those that can only drink so much Kool-Aid.