Credit cards are widely accepted, except obviously at street vendors. But you'll need cash, too. Take cash out from an ATM. No problem finding an ATM these days. For endless discussions about ATMs and exchange rates that will make your eyes glaze over, see threads on the Cozumel forum.
Are you doing any self-catering, or is the plan to eat all meals out? Budget of course depends on how well you like to eat. Actually, that's a terrible way to phrase it because to me, the cheapest food is often the best. I don't go for fancy restaurants or the trendy vegan coffeehouse smoothie whatever thing; just feed me traditional tacos and tortas. My weakness is ceviche and seafood cocktails, though, and those can go for 200 MXN. Also, are you budgeting for alcohol or are you a teetotaler? (I love my Bohemia Oscuras, but not when I'm there for cave diving.) If you're not drinking, then yes, 400-500 per-day would work. Anyway, since ATMs are the way to go, even if you don't take out enough cash initially, you can get more during the week. No worries.
When my wife and I travel for a week we take out (if I recall correctly from our December 2022 trip) 4000 at the Cancun airport ATM to get us started. We tend to make our own breakfasts and eat other meals out.
