You can get an umbrella policy potentially. But it might not be as good as insurance for a boat. It might be costly. specially if you have an expensive boat.

But like I said take current policy, and take off collision, medical, towing, and if your state allows it liability. At that point you would be at comp. If they dont let you take off liability then lower the coverage level to the lowest legal limit which should be like 25 or 50,000. You can always raise your deductible on comprehensive to like 1000. So in a sense this is modified storage insurance and do it every year on my camaro. Back in Mi they allowed storage insurance, now that I Live in NY they require liability dont matter what, so I have to play the game. Also if you have a loan they are going to want to see collision on it and potentially comp.

I am not too smart, but I don't think you can have an umbrella policy cover anything that is not covered by an underlying policy.

You are 100% correct. An umbrella policy is ONLY used to extend the liability limits of a primary (underlying) policy. Meaning, you can't drop the liablity on your boat policy and get an umbrella policy. If there the underlying policy is not in force, then the umbrella would be worthless.