The Eastern Caribbean dollar (EC$) is the local currency. Major credit cards are accepted at most hotels, car-rental agencies, dive shops and some of the larger restaurants. All of the major islands, except for Mayreau, have a bank and one or more 24-hour ATMs. But the network can go down, so carry backup cash. People will always accept US$.
There is 15% VAT that is added onto most retail items; this will already be included in the price. Most hotel rates have 10% VAT and 10% service charge added on top of them.
In restaurants a tip of 10% to 15% is the norm, though it's often added to the bill; 10% is usually added to hotel bills. A 10% tip is customary in taxis.