seek advice on offshore cruiser
Goes something like this......
Simply document back them that the boat is not located in Maine, taxes (show receipts) have been paid to the state where the boat is kept.
Its like owning property, etc. in another state - both states are not entitled to the taxes, only the state where the property is kept. If you ever move the boat back to Maine and Maine has a higher tax rate, then you will owe the differential (of current taxable value) to Maine.