Almost had a heart attack.

[Fr. Frank]

You should be able to improvise an inexpensive high water alarm by mounting a float switch in the bilge, a couple inches above the float switch for the bilge pump, and wiring it to the alarm buzzer for your Mercury engine.

You can also wire the float switch to a small 12v. electronic siren or bell, like a old fashioned phone bell, all available from Radio Shack. If your boat is behind the house, you may want to mount it under the T-top, or externally, so that you can hear it in the house.