Please see the Cenotes thread for more information about Aquatech - they are likely to rent you tanks with your certs. As well there are akumaldiveshop.com, akumaldivecenter.com, etc. etc. and even links from LocoGringo's website here to Akumal dive shops.

If I were you I would e-mail in advance to make sure you don't have to waste time scrabbling around for tanks.

And, yes, night, shore and solo dives are allowed. I don't have to share any precautions (other than not to leave valuables on the shore whilst you are diving!) with your level of experience. And I do tend to check the filling areas and conditions, as I really hate the idea of steel rust building up in the tank orifice or regulator, carbon monoxide, etc. and not everyone is as careful as they should be. (I've seen a gasoline-driven compressor in an enclosed room with insufficient ventilation, and had a reg crap out from rust dust from a steelie I was using.)

BTW, if you are going to be there this winter, there's an aggregation of female (many pregnant) Bull sharks in the area that a lot of people enjoy going to visit. (Personally, I look for folks that are not chumming and committing idiocies when wanting to do well-organized, non-harmful shark diving.)

56 years of scuba diving, 26 certs inc Rescue and still diving... ¡buenos buceos!