Janec, I'm not sure Reg Z defines real property. In order to answer your question, you would probably need to know on what body of water the slip sits? Public lake? Private pond? If public property, then slip might be personal property, and if private pond, then it probably would be a part/and affixed to the private real property. I would make sure it isn't just leased by customer. Is your concern for a boat that is being used as a primary dwelling?
I'm not sure I've helped. Perhaps someone else will comment...