Yes, that's how it works. Your fee covers use of the facilities, food and domestic drinks. There is usually an extra fee for equipment rentals (kayaks, jet skis, snorkle gear, etc.) and for premium drinks--although some of the "all-inclusives" actually mean "all-inclusive."
You can make a reservation yourself and provide your own transportation. Or you can book it as a shore excursion through the cruiseline. Another option would be to find 7-8 other passengers who want to go to the same resort and share a van.