If you are planning to park at BC/YC in the AM and spend the day at Epcot, then no.... it is not really appropriate, but there is no reason that you can't park there, if you are eating there and taking the cruise that evening.
If you are staying at a Disney owned hotel, and have a PS in the morning, I would take the bus to MGM, then catch the boat launch to BC. When finished, walk to the back entrance (International Gateway) and enter Epcot from there (it will put you in the park between Great Britain and France). When you leave, go through the same gate, and walk back to BC/YC. Eat, take the cruise, and when done, take a bus from there to DTD, and transfer to a bus back to your resort.
If you aren't staying in a Disney resort, drive to BC for breakfast, move car to Epcot, then when done, move car back to BC.
Ah, the travails of manuvering through Walt Disney World!