We are getting way off base.
Actually most of MikeM's comments on this subject are very accurate.
You are not going to burn up an alternator at idle. It just does not put out enough current at idle to make much of a contribution. If the RPMs are very low the alternator can't produce enough to get over-worked regardless of what the regulation would be.
The real danger to the alternator is when simultanously a large stereo is being played hard plus the batteries are in a deeply discharged state and the alternator is seriously loaded down at RPM. The 20 or so amps of the additional boat operations when underway are another draw that is piled on.
If you gave the alternator a break by placing the stereo in the limp or off mode while the deeply cycled batteries are getting a recharge, you could avoid the excess stress. Although many boaters don't want to run with their big audio system shut down....ever.
Running the batteries (even deep cycle batteries) below 12.0 volts is particularly bad for the batteries AND for the alternator.
Alernator current ratings are for a short term duty cycle and at higher RPMs.
It is pointless, and actually counter-productive, to have more depleted battery capacity than your charging system can effectively service....whether by the alternator, shore charger, or combination based on your individual scheme and usage.
Another valid reason for running all Class D amplification.