There are several ways you can get there, but none of them are direct.
#1 - Take a bus to Disney Studios (if still open) and then walk or take a boat to the Boardwalk.
#2 - Take a boat or bus to Downtown Disney, then a bus to the Boardwalk
#3 - Take a bus to Epcot (if still open) - Walk through Epcot to international gateway, then walk or take a boat to Boardwalk
#4 - Take a bus to MK (if still open) - then take a bus to Boardwalk
#5 - Take a bus to Animal Kingdom (if still open) - then take a bus to Boardwalk
#6 - Take a bus to Blizzard Beach (before 9pm) - then take a bus to Boardwalk
#7 - Take a bus to Typhoon Lagoon (if still open) - then take a bus to Boardwalk
#8 - Hire a taxi for less than $10.00 straight to the Boardwalk
Hope this helps!