Like every where else in the world, living in the Rivera, Playa del carmen, Cozumel, Cancun dependes on you, a little luck and paperwork to work legally here wich may be a pain in the neck, in Cancun and the Riviera there are all kinds of Gear, repair shops, and huge dealers and you can get anything you want, at the Dive op I work for we do have gear for sale and when a client wants something we find it here, although prices change, best choice for any gear here or anywhere in the world is
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I am an Instructor since 99 (diving since 89) I have worked at shops here in Cancun and in Calif.Usa.,I teach, sell, guide and help with the administration of the shop in high season I make a lot more than 400us a week. If you teach and sell at booths at hotels there is good money, I own a car, a very decent laptop (dsl) running hot and cold water, not a luxurious life but I do good (in the dive business you will not become a millionare). Cozumel like mentioned here before is a little different but I know people from Canada Instructors that make an awesome living no hutts, I also know DiveMasters that make the $ but dont have any and live in hutts.
In Playa del Carmen a good friend makes a salary of about 2000us he rents a decent 2 bedroom great location for 450usd. He has a car, dsl, computer, etc.
I know very many foreigners that come to live here some like my boss are American and have get acquainted with the life here wich is very different, you can get a diveshop to promote you immigration papers wich take a while and cost you. At the shop I work we have an instructor from Denver, just got married to a Mexican girl and is happy with the life here.
Hope this info is helpful to you