You have 3 options--boat, bus, or walk:
1. Walk via walkway through Epcot resorts (past BoardWalk) -- 25 minutes.
2. Bus -- 15 minutes not counting wait time.
3. Boat -- 30 minutes not counting wait time (you stop at Swan/Dolphin, Yacht/Beach Club, and BoardWalk on way).
Alternative route: take boat to Swan/Dolphin (8 minutes); get off there and walk across bridge to Boardwalk (you will see it as you exit boat), walk down boardwalk to Epcot entrance (total walk time 8 to 10 minutes; thus total travel time about 16 to 20 minutes).