Fastest is a taxi. They're readily available at all the resorts. Second fastest option is taking the Pop bus to the MK ~ hop on the resort monorail line and get off at the GF. As you're passing by the security check point for the MK, look to your left. That's where the resort boat launches are located. If you happen to see the GF boat there or just arriving, I'd bypass the monorail and hop on the boat.