They might try to trick you into paying if you anchor on the outside in Marigot, if you clear in at the ferry
terminal they will charge a "park fee" but if you go inside to clear in at the Capitanerie they won't; there a boat that circles around the Marigot anchorage that looks vaguely official and is staffed by gents who wear vaguely official uniforms - but the aren't official.
There is no charge to anchor on the French side of the Simpson Bay lagoon
. I was there last in May. The "border" is quite easy to discern as the Dutch side is (un)surprisingly empty while the French side is rather more crowded.
Note that the depths on the French side aren't great and there is frequent silting.