If your staying at the Catalonia riviera Maya in Puerto Aventuras:
Walk out of the resteraunt 5 paces and have a word with the chaps in the hut next to where you exchange your beach towel every day. I did 20 or so dives with them in November 2008. I did a 8 cenote dives, a day out to Cozumel and the rest in the house reefs.
First dive was around 8.30 am. Meet at the hut for 8am, wander down to the boat with personal kit- cylinders and weights on the boat in the marina. Second dive mid morning, afternoon dive around 2.30pm. Cenotes is a day trip -2 dives. Cozumel is an all day trip. Depart hotel at 7am not likely to be back until 7pm.
Temp wise I was in a 3mm shorty for the sea dives and I was fine. Temp was around 26/27 degrees. Cenotes I was in a 5mm full suit. Viz was great all round- as far as he eye could see basically. They have all equipment that wasn't really good order including hire suits.
There's also another operation down in the harbour at