I tried to ship a playboat (Necky Orbitfish) with Greyhound a few years ago. The boat said it would fit their regs. When I brought the boat to the bus station, the guys there told me it wouldn't fit in the back of the buses, regardless of what the Greyhound website said. Apparently, not all of the Greyhound buses are the same model, which plays an issue.
I found Airways Freight last time. Was something like $90 for ATL to BOS. I had to wrap the boat in bubble wrap, but it was still the best I could find at the time. They only ship to/from major cities though.