Regarding monies, you should bring both US cash and traveller's checks. Perhaps, small bills in cash ($1, $5, $10, $20) and larger denominations in traveller's checks ($50, $100, etc.) In AC they take both forms, but they will most likely give you change in Belizean dollars.
There are ATMs on the island, but none serve US banks which I am aware...only Belizean banks.
Yes, most credit cards are accepted in AC. I'm not sure about Amex. But, if you use your credit card, there sometimes is a surcharge. But, whenever I go, I ALWAYS use my credit card which allows me to bring less cash.
Don't know about Maruba...sorry.
Regarding clubs at night, I recommend beginning your evenings at Fido's (my favorite spot), then continue to clubs - barefoot iguana's, jaguars, etc. You should look at the main web page for additional info. It's fun for young folks..I'm young too! and I love AC.
Hope this helps.