fishfry once bubbled...
Just got back from CZM last week. I would only use Travelers Checks for your dives. The stores love cash, remember they only want to give change in Pecos.
In my experience, the shops on Cozumel have usually given me change in whatever currency I used for the transaction. Occasionally, they have given me pesos when I gave them dollars, but they nearly always ask "Pesos OK?" first.