The boats that serve DTD aren't the same as the ones that serve Epcot. The DTD boat loop goes to SSR, POFQ and POR. The boats that dock at Epcot go to Yacht & Beach Club, BW, Swan & Dolphin and MGM.
You could take the boat from DTD to SSR and bus it to Epcot, or take a bus to any resort and then hit Epcot. Maybe the best idea is to take a bus from DTD to Yacht & Beach Club, then walk to the International Gateway at Epcot. It takes less than five minutes.