One of the most common level-sensing switches is the float switch. ... but you're only concerned with the water level. The specific gravity of the oil is between 0.8 and 0.9 and that of water is 1.0.

QUESTION: My boat’s twin inboard engines are 350 cubic-inch V-8s. The port engine seems to manufacture oil! The oil level becomes higher after the engine is run. The oil looks normal. ANSWER:… ...