Since St. Maarten/St. Martin is a dual nationality nation, there are two official currencies: the NAf (Netherlands Antilles Florins) on the Dutch side of the island and Euros on the French side of the island.
On the Dutch side you could say that the currency mostly used is US Dollars and the Naf the least, only civil servants get paid in this money.
On the French side, the Euro is mostly used, but they will of course take your US Dollars.
There is no reason to change your US Dollars when you come to St. Maarten. It is however handy to know the exchange rate, since officially the stores must show all prices in Nafs. But again, all prices marked on St. Maarten are mostly in US Dollars with the Naf also listed.
The exchange rate is 1US$=1.78Naf This comes down to about 100Naf = $55.
Should you want to change your US$ to Euros for instance to get a slightly better rate than some merchants would give you on the French side, there are plenty Change agents in Marigot. Look for the Change Point signs.
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