One that I rarely see mentioned is Coral Princess. It's a bit North of town and a long walk but they had set taxi rates of 70 pesos to and from, at least that's what it was last year. They have a good on-site dive op called Pepe Scuba. They rent tanks (it was $15 last year!) and the shore diving is very easy. We headed South until we got to about 1600 psi, stayed along the shore/wall, and saw a lot of interesting critters. When we headed out, away from shore, we saw very little besides sand. Our AL80 tanks lasted almost two hours and we rarely exceeded 15 feet. We will be there again in about 3 weeks
Their dive boat prices are a little higher ($99, I think) and the boat trip is a few minutes longer and may be subject to availability if you are not staying at the hotel. A 1-bedroom, ocean-view suite is about $200/night but we do a time-share exchange and get it for about $200/week
A nice play to stay except for the distance from town and their restaurant is mediocre.
Marine World is South of town and is across the street from Casa del Mar hotel, just before you get to the elevated, over-the-road walkway, next to Atlantis Submarine. You can rent tanks, weights, and other gear. A couple of months ago it was $10 for tanks and weights. You jump off the dock and if you go South you will eventually encounter some coral restoration gardens. If you go North you will find artificial reefs, busts of famous oceanographers, coral gardens, a lot of snorkelers, Discover Scuba customers, as well as Snuba divers and maybe those helmet things that are sorta like hard hat divers but for tourists. The dives are mainly around 25-30 feet. Not an abundance of sea life but I have seen squid, rays, eel snakes, morays, etc. Definitely bring your DSMB if you head South from the dock--lots of boat traffic. There is also lots of boat traffic to the North but they are mainly snorkel boats and are expecting divers to be there. Marine World is the dive op for Casa del Mar hotel so their boat dives may be subject to availability.
No doubt Scuba Club will rent you a tank if you are not staying there and it's probably only about $7 or so, but the dive shop closes at around 5:00 so you might have to bring a padlock and use the lockers if you want to do a night dive etc. I have not done this particular shore dive but I've heard it's pretty good, especially at night.
