In my work as an insurance salesman, I hear this question a lot. The short and easy answer to this question is a resounding yes! You will need to buy a separate insurance policy for your boat to cover the shortcomings of the very limited coverage provided by your standard home insurance policy.
Your standard home insurance policy falls short in 2 main areas: property coverage and liability coverage. It will usually only cover you for very inexpensive and very small boats (those with only a small engine or no engine at all). Also note that while it is being towed or while stored on your property, it might enjoy limited coverage from your automobile and homeowners policy, respectively.
Under your homeowner policy’s personal property section, watercraft and accessories are covered for up to $1,000 only. Of course, this limit varies based on the specific wording of you policy and the losses covered will be limited as well.
You will also have liability coverage on a very limited basis from the personal liability portion of your homeowners policy. In a typical policy, for example, if you own the watercraft, you will have limited coverage for watercraft with an outboard motor that has less than 16 horsepower (any other type of motor can be up to 50 horsepower). But if you own a non-motorized watercraft, it cannot be more than 26 feet long.
As always, you should always refer to your specific policy as insurance products vary across carriers but these are the general guidelines to help you understand the limitations of not getting a separate policy for your boat (to learn more about these policies, see Boat and Watercraft Insurance: The Basics).