You should check what kind of insurance you need - it depends on how and where you use your boat.
If you’re using inland waterways
You’ll usually need to have ‘third party’ insurance for at least £1 million if you have a powered boat or a houseboat.
You may also need insurance for some types of unpowered boat, eg a houseboat - check with the navigation authority that manages the waterway you want to use.
You could be prosecuted or fined if you don’t have the right insurance - the kind of penalty depends on your navigation authority.
The Canal and River Trust website has a list of insurance companies that provide boat insurance.
If you’re using a boat at sea
Check the MCA guidance if you own a small commercial boat - you may need statutory certificates.