Not automatically. For your boat itself, you would be required to have coverage for the vessel separate of your car insurance. Most insurers have inclusion for a trailer included, up to a certain size (but every insurer company is different, so subject to policy exclusion). Once you get into mega yachts and large watercraft requiring heavy lifting to get them out of the water, different policies and coverages need to be considered.
In terms of liability coverage should an accident happen while you are towing your boat, the liability coverage for your car would extend for third party damage. It is important to note, the liability extends from the policy of the vehicle towing the boat – but always best to connect with your broker to confirm the details of your policy.