I agree there are no entry fees on a personal arrival by ferry.
Since the water taxi is bringing people in I am sure there is some charge for the boat to pay - it may or may not be equal to what they are charging - it used to be more of a convenience charge as the passengers did not have to leave the vessel.
The way that BVI C&I is currently dealing with the water taxi entries will likely cause the STT water taxis to eventually change their fee structure to reflect whatever services they are allowed to offer.