They will overload the ferry boats. I have seen people with advanced paid reservations refused boarding due to overcrowding . If you are planning on leaving tortola the same time as them, Either get to the ferry very early or plan on a water taxi.

S/V Scuba Doo
Lagoon 450, (part of the the Charter Caribe fleet (all inclusive-crewed charters) based in St Thomas USVI ).
A TTOL sponsor