The first thing to note is that you are in the islands and that schedules should be considered very vague representations of what the operator intended to do when writing up the schedule. The lack of reliability
and quality in the ferry services has been a matter of debate recently at the minister level in the BVI.
It is wise not to pre-purchase tickets, since that locks you into a specific ferry that might not be the next one running, and if the ferry you booked isn't running you won't get your ticket refunded or transferred to another operator, but will have to wait. Just get to the ferry terminal and purchase
seats on the next one out. On Saturdays the ferries from the BVI to the USVI are crowded in the morning, and the afternoon ferries from the USVI to the BVI are even fuller (the same people, but this time loaded down with purchases) and sometimes passengers are forced to wait for the next available one. The Petite Pump Room upstairs is a good place to spend the time.
Private charters are a bit pricey, but well worth it if you have a larger group or are arriving after the last ferry leaves (which I believe is about 4:30pm from CA, but I heard that they might institute a "late ferry" which leaves after that). How many people are in your group?