From the videos perspective, That horn seems to be too rapid of turning on and off to be a sensor horn issue as it should be a little more spaced out in time or become one solid horn at higher RPM. Was that in gear or neutral? Boat under way or at rest?
If the horn is really turning on and off that fast, then it almost sounds like you may have a loose power or ground wire possibly at the ignition key. If under normal circumstances when you turn the key on and the horn beeps once letting you know the horn circuit is working, then it almost sounds like the key would be turning on and off almost as if you where turning the key on and off by hand as fast as you possibly can...If the RPM's are up high then possibly you have a loose wire due to vibration is a thought.
Bear in mind that just because everything seems normal/okay on the outside such as no true overheat, have 12+ volts on the gauge or (multimeter?), oil level is full, not over revving, "That there could still be an issue with say one of the sensors being out of range whether true or false, have a shorted or open wiring issue someplace, failing ECM, loose wire/battery cables, grounded wire to the warning horn circuit enough that while you know all the normal things that could sound the warning horn is good and full, the ECM special sensors are telling a different story altogether and possibly the only way to find the issue quickly is to either buy or have the engine EFI system scanned on a special marine scan tool either from a boat dealer or buy one from Rinda technologies like this one,
http://www.rinda.com/techmatepro/
Not having a scan tool makes it harder to find issues like these but not necessarily impossible either plus having the correct OEM service manual on hand might just be able to walk you through a few of the checks for each of the EFI sensors to see if they are within normal working range or if any of the wires in-between the sensors and ECM are damaged by using an multimeter ohms tester all the while a visual is also needed to ensure all is connected and not backing/pulling out of a wire plug connector too.
Other than the horn sounding off, the engine seems to run normal?
Is your engine serial number handy?
Hope it's an easy solution for you, keep us updated and good luck.
