Very nice!There used to be a group that promoted urban farming called SPIN farming (small plot intensive farming) and they would rent people's yards in order to grow food. I always thought that was a great use for a dormant yard but yours sound pretty great too.
Make sure that your contract includes the fact that no bailment is provided. Otherwise, when some third party drives off with the boat, the boat owners may come after you to replace it.Otherwise, good idea!