You can take the Friendship Boats from the International Gateway of Epcot to the Disney Studios. The ride is 30-35 minutes because it stops along the way at the Beach/Yacht clubs, Boardwalk, and Swan/Dolphin. If you just miss a boat you would have to add maybe another 10-20 minutes. It is a nice and relaxing ride if you are not in a hurry.
The walk is around 25-30 minutes, depending on how fast you walk. As others noted, when you leave Epcot go left over the first bridge which brings you to the Boardwalk side. Someone posted a good tip about the short cut around one of the pools. It does save a few minutes from staying on the main walk which loops arounds several shops and clubs.