Now that you mention the time it takes to scuba dive, I'm reminded of one of my other hobbies that is dying alarmingly fast--ham radio.
Ham radio, like scuba diving, has gotten easier over the years, as you used to have to build your own very expensive radio. Now they are cheap and easily available, like the $30 handheld on Amazon. The certification has also gotten easier, in that it no longer requires morse code. If you understand basic electrical theory, you can pass the Technician exam after studying for a day or so. But, you still have to find a testing center, which is harder than finding a dive class.
Most people nowadays don't see any point to ham radio, which is why it's really dying out, unlike scuba diving, which is difficult but there is nothing quite the same if you want to see marine life up close. With Skype, cell phones, email, etc, it's a really hard sell to get someone to use a radio with low voice quality and antennas they have to string up and frequencies they have to match to talk to someone.
However, there is one important parallel: people who get into ham radio with a just-barely-get-it understanding, often don't have much fun as they really struggle to get the gear working right. They buy cheap radios, talk to a local net a few times, then they give up.
The ones that really stay in it are those who have a good first experience, a good understanding of how the gear works and how to do what they want to do, quality gear that works properly (I bought a piece of gear once that didn't function properly and I went off air for a month I was so frustrated trying to get my signal working properly!), and a community that they get plugged into.
For technicians, that's likely a local ham club. For Generals and above, that might be a net they check into. There are 2 local clubs that I am fond of, but it's interesting--one has pretty good meetings, but their on the air chats are boring as all get out, and no one is volunteering to help. The other club has mediocre meetings, but they have 2 on air chats that are very lively and all sorts of people check in. Then on the airwaves, you've got some real friendly people, and some real jerks, who really turn people off to the idea of chatting. Then there are people who only use ham radio to "contest," where they fight to make contact with long distance people and get a card from them in the mail confirming the contact. When a rare station gets on the air, there can be 20 or more people all yelling their call sign over each other until the contact confirms one of them, it's real challenging for someone new to get into that kind of thing. It's very similar to some forms of technical diving! Infact, hams squabble over the silliest stuff, like whether the phonetic for "z" is "zed" or "zulu."
I think Wookie hit the nail on the head about how dive shops benefit from providing community, when they hold events that get people in the doors, with the focus on being there, and the sales just happen naturally as an extension of that community.