I have had the BoatUS/ Geico policy for 5 years. I have never had a claim. Your query was a good one. I suspect that there may be policy cost thoughts (e.g., does being on the hard lower premiums?). I wondered if my policy has any exclusions while on the hard.
According to the policy provisions, the standard marine policy covers liability while the boat is on the hard under contract with the yard. No lower price was offered. If you permanently remove parts of the boat (e.g., an engine), the insured is required to inform the insurance company.