Every dive and every surf session leaves from the dhoni, not from the safari boat. It carries the tanks, the compressor, the boards and two crew, and it can sit in a channel mouth where the big boat cannot.
It also means the safari boat stays quiet and clean while you are in the water, and that you get picked up where the current puts you rather than where the plan said you would be.
The dhoni is the difference between a safari boat and a hotel that floats. It is not an extra and it is not charged for.