This is a pretty good explanation of what they charge and why.

Dolphin Fees

International Processing Fees

BVI international processing fees are $40 per person each way are are not included in our posted rates.

We often get questions about the "international processing fees" we charge here at Dolphin for guests travelling to and from the BVI. The most often asked question is, "Do we have to pay this if we take the public ferry?" The answer is, "yes, but not as much". Fees for visitors using the public ferry system travelling inbound are built into the ticket price, and going outbound have to be paid at the customs house prior to boarding the ferry. This "departure tax" is currently $25 per person. This fee went up in the Summer of 2014 by another $5 for non-residents only.

All private operators have to be different fees than the public ferry each time they enter and exit. These fees vary from company to company but the most normal amounts they charge international passengers is between $35pp and $45pp.

It is most important to note that the 'actual' inbound and outbound charges do not exactly equate to the $40pp that we charge. There are many variables affecting how much of a cost Dolphin Water Taxi actually incurs bringing 1 person into the BVI. The rates we are charged for each person we bring trough the customs house in the BVI and for each boat vary depending on the day or the week, time of day, (holidays and weekends are more). There is also an expense of clearing the boat in empty for pickups, which we must do before coming to get anyone, and clearing the boat empty back into the US in the event of BVI drop-offs. Lastly, there is a per person administrative cost in the completion of the US Customs and Border Protection Notice of Arrival and Departure. Its a labor intensive form that is contracted out and must be completed for each of our international boat runs prior to its voyage.