Sorry, forgot something. There are no buses to either DHS (MGM) or Epcot from the S/D. To get to Epcot, you have two options...you can walk, via the sidewalk, to the International Gateway, which is the back door to Epcot. That walk should take about 15 mins. Or, you can take the boat to the Gateway. The boat will stop at the YC/BC, the BW before going on to Epcot. It is usually much faster to just walk.
To get to DHS (MGM), you also have two options....boat or walk. The boat will go directly to DHS from the S/D dock...so that will take about 15 mins once you get on the boat. The boats come every 20 mins or so..if you just miss the boat, you may have a longish wait for the next one. You may just want to walk in that case. You would just cross over the bridge and head into the boardwalk area. There is a sidewalk that runs beside the canal (that the boat is on) that takes you to DHS. That walk will be between 15-20 mins depending on your walking pace.