I also came through same kind of problem few days ago.
Mercury oil tank sensor.
There are many complaints by the users of mercury motors.
This video will show you what causes this problem and how it can easily be corrected.
You can fix the problem by replacing wire to the horn for solving this particular problem or by replacing the sensor that measure the oil level in the motors.
The engine 2 stroke oil tank has a low oil sensor in the bottom of it and can t be replaced without replacing the whole tank.
The oil level is full and you replaced the oil level switch but the problem will not go away.
So your mercury outboard is experiencing the common problem of the alarm beeping 4 times every 2 minutes.
Low oil sending unit stuck on creating alarm from control panel.
Mercury oil tank level sensor 4 beeps every 2 minutes.