Sounds like a great deal, but I've got to ask ....
What is your marina's policy about 'filling' empty slips, including those under contract - (like the one next to your friend's houseboat) with transients?
I know my slip contract has this paragraph - grammar is there's not mine
. In the event the vessel of Vessel Owner is to be absent from the marina for a period greater than two days, whenever possible, Vessel Owner shall provide two-days advance notice to Marina of the departure date and the intended return date, so that the slip can be made available to others. Vessel Owner should provide a written float plan to Marina or to others. Marina may temporarily license, rent, or otherwise use any empty slip at its option."
Yes, it seems like a really good deal for the marina, and we found it common to every slip contract we read while looking for a new home marina after we retired a few years back.
So, while it seems like you've got a great 'free' deal - you might want to ensure you're not going to get a knock on your door from the marina staff asking you to move or pay up.
Again, if it's a good deal, good for you, but at least be forewarned so you don't run into problems later.