When we were at the GF in December they had a water taxi service that took us to Fort Wilderness from the GF marina. It cost $20 and it was well worth it. The taxi service takes you to any resort on the lake, CR, Poly, WL or FW. Or it will take you to MK. But it is a one way service, GF to your destination. It won't pick you up again for the return trip to GF.
But if you want to travel using free transportation then I would take the monorail to MK, then the WL boat. You can also take the monorail to CR and get the resort boat but I think there is more walking using that route. I would give yourself 40 minutes using this route, although it could be faster depending on how long you have to wait for the monorail and boat.
The resort boat, which is the boat between CR and WL, runs until 11pm. The boat between the WL and MK runs until one hour past park close. You can also take the WL bus to MK and catch the GF boat or the monorail to get back to GF after dinner.