You could try unpluging the too wires that go to the stern drive gear lube bottle mounted to the engine then wait about 60 seconds to see if the horn stops going off every 60 seconds. Then also the water pressure sensor might by picking up a bad reading some place, like low water psi, air bubbles, Or in some cases the water sensor it's self might be getting a bad ground and sending the ecm a out of range reading, try looking at the water sensor and see if you can see a lot of extra paint a round the sensor, And if you have some extra wire with you try touching the sensor it's self to a good ground on the engine.