The price you see advertised (the charter fee, say 10.000 €) is just for renting the yacht and paying the crew's salaries. Think of it like booking a super cool floating hotel.
What's NOT included is almost everything you'll use on the trip. These extra costs are covered by something called the APA.
What's Extra (covered by the APA):
Fuel: For the yacht, for the tenders (smaller boats), and for the cool toys like Jet Skis. Motor yachts use a lot of fuel!
Food and Drinks: Everything from gourmet meals cooked by your private chef to snacks, champagne, and sodas.
Port Fees: If you want to dock in popular, fancy marinas (like Monaco or Saint-Tropez), you have to pay a parking fee for the boat.
Taxes: There’s a sales tax, called VAT, on the charter fee in Europe, and the rate changes depending on the country you start in.