There is no test requirement to fly to the US mainland from the USVI. It is a US territory. The testing requirements for US entry only apply to international arrivals by Air. Flying from the BVI to the USVI or Puerto Rico still require testing. Under current US rules if you arrive by sea to the USVI and fly out to the mainland US no tests are required.
That assumes the USVI does not decide to enact a USVI only rule as they did with the portal and 5 day testing requirement. As I read it now if you ferry from the BVI to the USVI and fly home no tests are required.