I've just started diving early this year, and as I live in Guadalajara, the nearest place to dive is Manzanillo (the only place I´ve dove, in fact), I've done all my boat dives with Neptune´s Diving and they´ve been just great with me, because of the cold they lend me a full wet suit (mine is short) and weights without paying them anything, and they know great dive spots, like Cinto de Cobre, Carrizales (aprox depth: 50 - 60 feet), Piramides and Orejas de Caballo (aprox depth: 80 - 90 feet), . the DMs are very friendly, specialy Lluvia (spanish name for Rain) and Sean.
When I dive from shore, I get my tanks from them, just one word of advice, make sure your tanks are completely filled, because sometimes my tanks are between 2700 and 2900 psi.
They now have the boat that someone (sorry I don't remember the name) talked about, and its a nice one.
About Susan and Carlos of Underworld Scuba, i have no experience diving with them, I´ve bought some stuff from them and once I rented them the tanks, they always check that your tanks are completely filled, but they are expensive than Neptunes.
And finally one good news for you, Neptunes Diving is now working at the Karmina Palace, under the name Offshore WaterSports, so if you stay at the Karmina, you can get all the stuff you need from them without going downtown, and the boat is right there, in the hotel's beach, so its more practical.
Finally, if you want to make some Coast dives, I can recomend you to places to go.
1.- Club De Yates Beach: the place is horrible to go and spend the day in the beach, but is a good place to dive, specialy at some rocks that are at 200 -250 meters (uhm.. i think that is between 650 - 800 feet) from the coast, i specially recomend it for Night Dives (Max depth: 30 feet around the rocks, i think it gets deeper after that).
2.- La Boquita Beach: It´s usually filled with locals enyoing the beach, but there is a ship wreck (or something like that) in shallow water with some great life around it, but the bottom is muddy and if you go with inexperienced divers your visibility will drop. (max depth: 20 feet). (by the way, my buddy lost his wallet diving there, if you find it please tell me, hehe)
I think that if you ask any of the LDS about other coast dive sites, they will tell you about them, those are the two I know.
I Haven't much experience diving, just finished my AOW, but I hope this helped you, have a nice trip and enjoy México.
Neptune's Diving:
http://www.neptunesdiving.com/index.html
Underworld Scuba:
http://www.gomanzanillo.com/scubamex/index.htm