From HS to Epcot's International Gateway you can take a boat or walk. I do also think there is a bus from the front of Epcot to HS.
But all the parks are mostly directly connected to each other by bus. The exception is to go from the MK to Epcot you take two monorails.
Also in the past buses to the MK dropped off at the TTC but now take you to the front gate.