How should I collect the balance?
However you normally take money — cash is most common here, and pesos are easiest for everyone. The exact amount owed is on the booking.
Guests are told before they book that the balance is paid to you on the day, and that cash in pesos is usually preferred. They are also told the exact figure, so there should be no surprises.
If you take cards, say so in your description. It is a genuine advantage to a visitor who did not want to carry a large amount of cash to a marina.
Quote the balance in the currency the guest sees on their booking. If the trip is priced in dollars and the guest is paying in pesos, agree the exchange rate before the boat leaves — a rate written on the booking or in a message beats an argument at the dock at the end of a long day. Take the balance before you cast off, not after.