Charlie: The "Liability" portion of a boat's insurance generally covers you for events
which are related to the operation or use of the boat. For instance, if some falls down on your boat, breaks an arm, and then sues you, or you damage another boat while you are docking
The boat's insurance will normally not cover you for land-based events
. The clearest example is anything related to the operation of a motor
vehicle. (The insurance company doesn't want you to have "free" auto insurance on any of your cars. Of course if you rent a car or borrow a friend's car, with permission, the car's insurance should cover you.)
Other land-based events that your boat policy will not cover are claims that you injured someone, or their property. For instance if you are pushing a shopping
cart in a store and accidentally knock down someone, breaking their hip, or you are riding your bike and collide with a parked car, damaging it.
It is for those land-based incidents that you may very well want to have some kind of separate liability coverage. The real problem is that for true liveaboards, it is difficult to find an insurance carrier which will cover you if you don't own a house or are a tenant in a leased premises.
The descriptions of "coverage" above are gross generalities and everything depends on the actual policy language.
Your best solution is to locate a good local insurance agent and see if they can find some way to obtain coverage for you. It won't be easy - this is a perennial problem for liveaboards. You could also go to the company or agent who sold your boat policy, and see what they might have available.