The boat runs between the resorts every 20 minutes or so. There are actually 2 boat routes available...one between the WL, FW and MK and one between the WL, FW and Contemporary. Since you are going after 3pm I would take the WL, FW, MK boat as it will go directly from the WL to FW. The other boat will stop at the Contemporary before it goes to FW.
Just an FYI...there is also a bus route between the resorts. It runs every 15 minutes or so and will drop you off on the opposite side of Pioneer Hall from the marina.
Have fun!