Don't know for sure, but I think you can take food along.
One thing, however, it's a bus, there's no tables, or good place to clean up if you spill stuff. Why eat a boxed lunch when you're about to board the ship with tons of food onboard (included in the fare)?
We typically get up, finish packing up our checked bags (for pick up by 8:00), go get a good breakfast and then just "hang around" until time to get on the bus.