Unless otherwise stated in the advert, fuel is not included in the rental price.
At the end of the charter period, you will therefore have to refuel the boat and return it with the same level of fuel as at the start of the charter, or pay the owner back at the port. Some owners offer a fuel package that you can pay for in advance.
In the details and conditions of the boat, you can find out whether fuel is included in the hire.
The owner will be entitled to charge you for the fuel at the end of the charter if it has not been topped up.