Since there seems to be a power loss, Make sure all the power electric wires are clean and tight and also using a multimeter, check for loss of 12 volt power such as the alternator as you may have an intermittent power loss or short circuit going on possibly causing the beginning key on alarm to sound intermittently.
Also try disconnecting both wire harness connectors going to the thunderbolt 5 ignition module and ensure the wires and pins on both sides are clean and not backing out of the connector.
Plus next time you're on the water and the warning horn sounds, try disconnecting the wire connectors to the ignition module seen below for a minute then run the boat again and see if the warning horn turns off. If not, disconnect the engine temperature sending unit with the yellow and black wires and run the boat. Then disconnect the 2 wires going to the gear lube reservoir....If not then there's only two things left which would be the engine oil switch or a short to ground on the Tan/Blue wire somewhere a long the line.