I've taken a boat along w/a houseboat by putting a boat trailer in a basement under the houseboat, like:
https://live.staticflickr.com/65535/46990357105_2ccd8a1727_c.jpgI doubt, even if you found a way to get a post or cleat actually on the houseboat, that there is coding that would allow a boat to be towed along with it (which I think is the point). In case you're wondering, sims could use my basement boat similarly to how they use cars in backpacks. I think they were inclined to leave the boat in their backpack afterwards, though, so I would put it back on the trailer myself. I don't know that this needed to be in a basement - I just didn't want to use up space above deck for it.
(Edited to add: Not sure how I got the blue type - must have picked it up from my picture link somehow.)
