In addition to the NPS moorings in St John (only $26 or $13 with the Senior Pass), there are many free overnight moorings available in St Thomas - 20 at Christmas Cove, Little St James, and ~12 at Buck/Capella Island.
Head to Culebra in the SVI and you'll find 50+ free moorings throughout its many anchorages.
The free moorings you speak of are put there by the Department of Natural Resources and serviced by them.
