To get to CM's from the BC, I'd take the bus to the MK and then walk or monorail (one stop) to the Contemporary.
But you can indeed take the monorail from Epcot. First, don't take a boat to Epcot from Beach Club. It's almost an equal walk from BC to the YC dock to catch the boat as it is to just walk directly to Epcot's International Gateway. Second, once in Epcot you have to walk to the front of the park and out the turnstyles to get on the monorail. Third, when the monorail drops you at the TTC, switch over to the Resorts monorail and go 4 stops to the Contemporary.
Coming back, its one stop from the Contemporary to the TTC, then switch to the Epcot monorail. At Epcot, walk through the park to the International Gateway exit and back to BC.