There is a boat that travels between PO and DD, and you can board at either location.
It runs daily, but be sure to check on the schedule. This line has no other destination.
The boat from Epcot goes directly to Boardwalk. There is also a boat between Boardwalk and MGM. The Boardwalk boats run continously throughout the day. There are other boats around WDW as well. There's LOTS of fun and free things to do around the World!