The dreaded mercury alarm!!! Hard to figure them out as they all have a similar sound and function.
Does it go in to "limp" mode when the alarm goes off? One thing to check on those motors. If memory serves me, there is a "pre-filter" screen in the water induction prior to the water hitting the impeller. Many boat mechanics with this model don't clean them. Pull the screens off your lower unit (where the water is sucked in). IF for any reason those are clogged, the system in these motors will sense low water pressure. The alarm you are more than likely getting is a low water pressure.
They also have a similar alarm for low oil and about 10 other things!
If memory serves me, I think that year of motor had a recall on all of the cylinder coils as well.
I have OCD "Obsessive Crappie Disorder"