This morning I was able to get information on COVID-related entry requirements into the SVI.  I called the number listed on 
https://travelsafe.pr.gov and was told that to enter the SVI by sea on a chartered sailboat from St. Thomas, every person on board would need a negative result from a PCR test taken within three days of arrival in the SVI.  And every person on board would need to complete a travel declaration on 
https://travelsafe.pr.gov.Then, per 
https://usvitravelportal.com and 
https://usviupdate.com, to get back into the USVI from the SVI, every person on board would need a negative result from a PCR or rapid test taken within five days of commencement of travel to the USVI.  And every person on board would need to make a submission through 
https://usvitravelportal.com.So, that's not very attractive from a logistical convenience point of view.  The 
https://travelsafe.pr.gov person I spoke with did say that the SVI entry requirements could change between now and my December charter.  But I don't hold out much hope that it will get easier.