Just want to give a special mention to bviboater and Jorgen for the info about multi entry visas and that the fact that the Visa Waiver Program does not apply to non-bonded carriers.
Funny, this link at noonsite which was last updated October 2014 Puerto Rico Formalities claims the VWP applies to international crew arriving "for the first time". They say it has been confirmed to be perfectly legal if they first take a ferry from BVI to USVI then they can enter the USVI again or SVI by yacht, as long as they have a valid ESTA (Electronic System for Travel Authorization). I don't think we'll try that but I'll mention it so if other international travelers are interested they can research.