Thanks Bill and gtod.
I forgot to mention that all 6 of us are EU (Slovene) citizens and US VI is off limits without a visa. None of us have a visa.
It is kind of stupid however that we can fly to US (visa Waiver) or travel with a ferry, but can not sail there.
I was thinking to ask in St Thomas if we can apply the rule of "visiting adjacent islands" as part of Waiver programme. We would already be in US and would just leave for BVI and come back. I am not sure if it works.
Edit: here is what made me think we could enter with a boat without a visa:
From: http://immigration.about.com/od/visa...a/visawp_2.htm
Q: Can a VWP applicant for Admission Be Readmitted To the United States Follwing a Short Trip To an Adjacent Island, Canada, or Mexico?
A:
* Generally, VWP applicants admitted under the VWP may be readmitted to the United States after a departure to Canada or Mexico or adjacent islands for the balance of their original admission period. This is provided they are otherwise admissible and meet all the conditions of the VWP, with the exception of arrival on a signatory carrier, in which case the inspecting officers have the discretion to grant the applicants entirely new periods of admission.
* The VWP applicant is admissible and may be readmitted to the United States under the VWP after a departure to Canada or Mexico or adjacent islands provided the person: 1. Can identify an authorized period of admission that has not expired,
2. Plans to depart the United States prior to the expiration date of their period of admission,
3. Presents valid, unexpired passports which reflect admission to the United States under the VWP, and