“...do I get committed to becoming a certified diver or just do the one-day tourist lesson and dive?”
There’s a hard-core diving group in Vegas, I think. Unfortunately, the sports shop (Sports Chalet?) that was really supportive actually went out of business a few years ago so things have probably changed a lot since I dove Lake Mead. You still may be able to make some connections on Facebook—they had an active fb group a few years ago. And maybe another dive shop has stepped up and become the leader in the SCUBA community around Las Vegas now, too.
If you want my input, I would suggest you think seriously about whether you are inclined to enjoy dive travel because Lake Mead leaves a lot to be desired when it comes to SCUBA sights. If you get certified before your trip and then do some diving in Belize, you are going to want to follow that up with more diving at other SCUBA-cool places because the underwater sights in Belize are really wonderful. That will whisk you into a new kind of travel—and probably expand your photography interests as well. You will also be very tempted to invest a boatload (ha) of cash into a lot of new equipment - for SCUBA as well as for underwater photography.
If you just want a one-time experience diving in Belize, then the vacation training will probably suffice for that purpose. You’ll have to fight with the beat-up equipment and the gear that doesn’t fit quite right—the fins that are too small and the face mask that gives you a sinus headache because it’s too tight, but it still leaks—all that fun stuff. (Gah!) But If you get past all that crap and get hooked, you’ll probably need/want to sign up for more serious training back at home after you return. Certifying on vacation in a resort pool isn’t really as rigorous as the weeks-long training experience you get at a local shop where you make connections and become one of the groupies. There are dive masters at the resort who are supposed to walk you through the science, but they know most vacationers are just wanting to see beautiful things underwater and aren’t really in a frame of mind to digest all the science while they are on vacation.
One more thing to consider—SCUBA is a really interesting socialization hybrid. It is absolutely essential to always dive with another diver (a “dive buddy”)—NEVER dive alone. That can be challenging for us introverts. However, once you are under the water, there is very little communication other than safety checks, so diving itself is really a fantastic activity for introverts. In my experience, there’s nothing else quite like it.
That’s my two cents... :)