How do I determine if its
the
alt. or the regulator.

With a progressive test light or (better yet) a meter.

With the engine running, measure the voltage at the connections on the back
of
the alternator. Should be well over 14 volts.
If it isn't, (and especially if its the same reading you're getting off your
battery terminals when the engine is not running or there is no charger
connected),
look at the alternator.

Thanks to everyone who responded. I used a meter to check the battery voltage
with the engine off and noted 13v, I then checked each battery with the motor
running at idle, and got between 13.6 and 14v, I then checked the alternator
with the meter, and got a reading of 14.4v at idle and 13.8 at 2000rpms. I took
the boat out and ran it for about 20 minutes at 3000rpms. I watched the
voltmeter on the boat and on my sonar go from 13.8 down to almost 11.6 volts,
as soon as I would return to idle it would go back up. I was alone and didn't
think it was a good idea to leave the helm at speed to check the back of the
alternator with the meter, so I have only the two readings from the boat's
meters. At 3000rpms (or any rpms for that matter), are the meters on the boat
reading battery voltage or charging voltage? HELP!
John