"Is it possible to "sub-let" someone's slip? Say boat-owner X owns a slip"
Anything is possible IF the slip owner really owns it in fee simple, without any restrictions in the title, without any dock-o-minium restrictions, without any complications that prevent them from leasing their property out.
Is it possible? You'd have to ask the mystery owner to check his deed for restrictions. And then, some folks just do it on the sly anyhow.