Hi LoneWolf,
I live in Puebla, 2hrs away from Mexico City towards the East. Last weekend I was diving in Veracruz (3hrs further towards the East) and could see many divers from Mexico City and Puebla there. It is the nearest sea destination for us. High season is Easter to mid-September, but there are dive shops open all the year around. Last weekend we had about 10m visibity on Saturday, and about 3-5m on Sunday. The DM said it can be much better during high season. We saw awesome marine life, including large marine life: a ~2m long eal, manta rays, and a ~2m long grouper that blew our minds. Also lots of nice coral formations. It is not Cozumel, but we felt it is a very worthwhile destination for the weekend, and plan to repeat next weekend. We (husband and I) are also novice divers: 30 dives so far.
Hope you enjoy Mexico!
Itziar
P.S.: We have an old car, and there are people who trim those driving times by 30%. The road is a well paved motorway. We never do more than 120 km/hr. Also people are not shy of driving long distances here: it is a big country!