Bring US$400 between the two of you, an ATM card each, and a couple credit cards of type MasterCard or Visa each. You'll be fine. If you plan to gamble, add that to the suggested amount.
This will pay for sundries at the hotel beach shop, tips, taxis, a couple nights outside the resort, perhaps a nightclub or dinner, a couple motorized water sports events, and some souvineers.
Anything extra you can get using your ATM cards or credit cards via cash advance at the ATM of walk-in at any bank.