Upon entering SXM wouldn't proof of taking the boat be enough? When the vessel enters British possession their will be another check-point where documents must be shown. Then how will you get from BVI back to Texas?

Not sure this is the same thing but we had a friend come to SXM on a cruise ship, abandon the rest of the cruise itinerary, stay with us a week bought a one-way ticket back to USA in SXM with no problems.