There are different types of enchanted boats in the Harry Potter universe.
The arriving first-years travel across the lake to the castle in a flotilla of small boats, propelled by magic (PS6).
The Horcrux cave contained an enchanted boat, hidden underwater such that "none but a very great wizard could find it" (HBP26). The boat was very small and glowed green. Dumbledore believed that it had been enchanted so that the weight it was expected to carry was not relevant, but rather the amount of magical power in its passenger(s), possibly with some emphasis on age and/or experience (HBP26). Like the Hogwarts boats, this boat moved without the help of its passengers (HBP26).