Do You have a larger boat too or is it just the dingy? If it's just the dingy then I'd say minimum 3rd party insurance. However, in the event your engine falls off the transom whilst fitting or removing and it takes a swim, then you'd wish you had fully comp.
If you have a larger boat, list the dingy and outboard as part of that boats insurance. But as @Andy135 says, there is no need by law.