I spawned a boat on my survival single player on Minecraft 15w33b because I wanted to move (stupid world gen put me on a remote island) I docked my boat in hopes of a place I found and I didn't like it, so I wen't back and tried to get on my boat but it wouldn't let me in and I could stand on it and I couldn't move it as if it was a solid block.
I believe that this is a de-sync issue. This means that your boat is not really located where you see it.