It is probably the sending unit and we suspect someone loosened that end of the boat reservoir cap when filling, possibly jamming the arm on the float in the full position. Go to the wire end of the boat reservoir and loosen the cap, slip the center piece up an inch or so and see if you can rotate it to reposition the arm. If not that, then it is the sending unit we suspect and that is a warranty item, IF your engine has warranty remaining. On engines covered Mercury does cover all Mercury components through duration of that warranty on the engine. This changed from a one year component warranty in the late 2000 era.
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