As Marci said, the reference thread on DMX has a LOT of information in it.
Sidemount tanks are not hard to source down there. I love working with ZG, because they're friends as well as being a top class operation, but depending on when and with whom you go, you might find it better to stage out of Tulum, in which case ZG isn't terribly convenient for gas. Cave Heaven, the UTD operation down there, has built some nice lodging for divers, and their current prices are VERY attractive. Given the Z-system, I'm sure they'd have sidemount tanks available.
I second the recommendation of Jason as a guide. He's a good friend and a REALLY fun guy to dive with. You can reach him at
Essential Scuba Training. Steve Bogaerts is, of course, the go-to guy on sidemount, but he's not always available and I think he's more expensive. Of course, if you stay at Cave Heaven, you could use them for guiding, and I think they may be running some package deals with lodging and tanks that make their otherwise rather high prices look better.
I have yet to find a BAD time of year to go to MX, although I haven't really gone during hurricane season. The winter is, of course, cooler and less humid, but it's also high season and prices are higher. I have been there in March, April, May and June, all of which were pleasant, although it definitely gets hotter in the latter two months, and there are more mosquitoes. I've also been there in October and November, both of which were very pleasant from a temperature standpoint, but we had more rainstorms. Only once have I seen enough rain that it would have impeded cave diving, though, and that was a torrential downpour that lasted more than 24 hours and made getting back to the airport very interesting.
Marci is right about the money. A lot of places only take pesos, and don't count on credit cards being accepted in any kind of small business. (The gas stations and dive shops I've been to all take them, but restaurants are spotty.)
Which caves? ALL OF THEM!!! Honestly, you can't go wrong anywhere. Since you haven't been there before, I'd stick to the major "tourist" caves, because all of them are beautiful. A good place for a "shakedown" dive is Taj Maha -- big passage (in fact, some HUGE rooms) and lots of halocline effects, and you can just swim the mainline (which is pretty easy to find). For a second dive, you can do the cavern line and jump off at the end of it, and swim that main line. It's not as decorated as some of the other caves, which makes it a good place to start.
Grand Cenote can keep you busy for a week all by itself. The Cuzan Nah loop is beautiful, and the jump off it to the Caverna de los Colmillos is stunning. The Paso de Legarto line leads to the jump to Much's Maze, which I think is fantastic, and also curves around and intersects the lines from Calimba and Box Chen. With your gas consumption, you will see a lot of this just on two tanks, although some of what I'm describing would require a stage.
You HAVE to dive Jailhouse while you're there, but do that with Jason. The cave itself is easy navigation, but the logistics of getting there are a little complicated.
Mayan Blue has some beautiful blue-and-white passages, and A tunnel is very decorated, but you have to have a bunch of light in there to see how incredible the spaces are. I really liked the little bit of the Dead Zone passage I saw (don't ask me why it has such an awful name).
I could go on. There just aren't any bad places to dive. Naharon, NoHoch, Dos Palmas, Dos Ojos, Minotauro . . . You won't run out of things to see in a week.
Get out of the jar!
