Are you planning to pay for your lodging and diving in cash? Dive shops tend to set their prices in dollars, so there's no advantage to paying in equivalent pesos, and if you put your hotel on a credit card, that's a done deal, so that leaves you with exchanging dollars to pesos just for "spending cash" for meals, taxis, and miscellaneous stuff. My point is that, to put things in perspective, unless you're a big spender, you probably won't be exchanging enough cash for differences in the exchange rate to make a huge difference. A few dollars more or a few dollars less will be a tiny fraction of the total cost of your trip.
As others have said, the most convenient way to get spending cash is using an ATM. Your card from home is probably going to be fine. Adding an extra layer of protection against fraud and ATM fees by using a card such as Schwab's is icing on the cake.
As others have said, the most convenient way to get spending cash is using an ATM. Your card from home is probably going to be fine. Adding an extra layer of protection against fraud and ATM fees by using a card such as Schwab's is icing on the cake.