Quickest way: Cab or your own car
Other ways, depending on times of day:
Monorail to TTC, take Epcot monorail to Epcot, walk all the way through to the International Gateway, walk to Boardwalk (that would be long)
Bus to DHS, walk to Boardwalk (that would be best using Disney transportation)
Bus to DHS, boat to Boardwalk (will take longer than walking from the bus stop)