You should check out Beerman's excellent trip report in the trip report archives, the first section of which is here

The area of Nettle Bay is a lot different 'vibe' than Simpson Bay, which is not to say it is bad, just different. It's a more residential area and many of the people there will be locals, as opposed to visitors. The beach itself there is not particularly good, in that there is often glass on the beach, so beach shoes are a necessity.

Businesses in Marigot will accept Euros and Dollars.