A private charter means the vessel is yours for the booked hours: your group, your music, your route around the harbour. The crew handles the boat, so nobody in the party is responsible for navigation or mooring.
Catering, drinks and extras are arranged around the event rather than set by a package, which is why every charter is quoted rather than priced from a list.
No other groups on board for the hours you book.
The vessel is operated by its own crew; you are a guest on board.
A chauffeured car can collect the group and meet the boat at the other end.
Three things move the cost of a charter: the vessel, the number of hours and the day. A weekday afternoon on a mid-sized boat and a Saturday evening on a superyacht are different bookings entirely.
Catering, beverages, and extras such as a photographer are arranged on request, so the quote reflects the event you actually want rather than a package you have to fit into.