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Your battery is grounded to the boat and the ground on the engine is bad so any voltage is going through the boat and then to the engine.
Disconnect the battery. Remove the positive and negative wires from the engine. Clean the connections and cables throughly with a file and wire brush. Reconnect the cables to the engine. Connect the cables to the battery. Put a volt meter on the battery read the voltage, around 12.5 volts. Now repeat the reading on the connections on the engine. They should be the same. If more than .5 volts less check the connections again and connect the cables. Make sure the key is off.
Let me know what you find, post back and we'll go from there.