A guest turned up with more people than they booked. What do I do?
Do not exceed your certified capacity, whatever the guest says. If you can safely take the extra people, charge for them; if you cannot, they do not board.
This happens, usually without any bad intent. Handle it at the dock: if the boat and your certificate allow it, take them and charge your per-guest rate. If they do not, the extra people stay ashore.
Nobody at this company will ever ask you to exceed your capacity to save a booking. If a guest pushes, tell us.