That is a good point, as i live in ohio so i am not close to any ports. I was trying to find a port near a train station since we have an amtrak here. As far as ships, it seems that the really nice lines offer these, in fact the one i saw that made me want to do it was on the NCL Breakaway.
I guess it's somewhat a good thing i do not live by as port as i would probably start doing these every weekend!