The currency used in St Martin is the Antillean Guilder (ANG). But most prices on the island are shown in US dollars. Once you leave any of the villas for rent in St Martin, you can use US dollars throughout the island, and you will generally get change in US dollars too.
ATMs around the island dispense USD. Credit cards are widely accepted, so besides smaller casual beach shacks or general tipping where cash is helpful, you can usually swipe and go.