Yes, even boats need insurance. Your homeowner’s policy and auto insurance policies will only cover a portion of the risk that owning and operating a boat entails. Most likely your auto policy provides coverage while the boat is being towed, of course this is only up to the limits on your auto policy.
Considering boats have the ability to be “driven” even if on water, they are a liability risk. Not only that, many boats are very expensive to purchase so you would be best served having physical damage coverage on your boat as well. Keep in mind though, depending on your insurance carrier, state of residency, etc, some policies have a “lay-up” policy or coverage will be suspended for several months during the time that boats are not typically in use. There are also specific areas where policies extend coverage: ie. inland areas (lakes, rivers) or coastal areas, and of course there are policies to cover those boats that are on the open ocean.