Is it possible that you have a pin hole leak in one of your sea water hoses, or maybe a hose clip that is not quite as tight as it should be allowing water to escape. Suggest you wrap some absorbent tissue around the length of the nearest sea water hose, give the engine a run for twenty minutes then check the tissue to see if it is wet. If it is wet, that might give you a better guide to where the leak is located. Maybe spend some time watching the engine while it is running to see if you can identify where the water is coming from, just keep your fingers away from any moving parts, fan belt, pulley wheels etc.