We've had guests arrive in SXM on one-way tickets to help us on a crossing. We sent them a letter, made up a letterhead for the boat advising Customs they would be leaving on our cat with us. I signed as owner/captain and they whistled through C&I.
If you can nail down your ride, a letter like this may help you when you arrive, PM me I'm happy to send you what we used.
Similar situation in St Kitts when we took on friends for the trip to Guadeloupe.