wrote
How long can a Mercrusier 3.0 idle out of the water
before there is any serious damage. My father-in -law
pulled MY boat out of the water with the engine still
running at idle speed (hearing aid off), and it was
about 2 minutes before someone came to the rescue.

A few minutes at idle isn't likely to have hurt the engine, but if you have
the usual rubber impeller type raw water pump, it may have damaged the
impeller. I'd replace it as a precaution; it's not a big job unless you find
or break something else in the process.