At the end of your trip, often before you get back to the port, the bill is drawn up. Everything you have taken on board has yet to be paid for, although there are trips where almost everything is included in the price. Additional costs can come from drinks, a shirt you bought, excursions, using nitrox, a bigger tank, renting diving equipment, taking a course or having a private guide. Don’t be surprised and ask for the costs of these things before booking. Some tour operators offer you a discount if you book it in advance, but pay on board.
When the bill has been prepared, it will be checked with you and you can pay it. As a rule, they accept cash in dollars and/or euros and the local currency. Payment by credit card is also often possible, but don’t be surprised if they charge for this. It is wise to inquire about the payment options in advance.