I think your options would be to boat to MGM, then take a bus to the Epcot entrance. I know there is a bus from Epcot to the MGM entrance, so I'm almost positive it goes both ways.
You MIGHT be able to take a bus from an Epcot resort to the Ticket and Transportation Center and then monorail from there.
To walk through Epcot you must have a ticket, UNLESS it is late in the evening and Illuminations is over, a CM might let you walk through Epcot to get to the front.