Do I need a second battery?

Are you using a deep cycle or starting type of battery? If it is deep cycle does it have at least 650 CCA?

Sure. You will charge your battery better/faster at idle with all the high power requirement accessories turned on. You can definately draw more power at idle than some alternators can put out when you have several added accesories all turned on at once.

Don't forget that high output alternators require more HP to turn them.

Yes, you got it exactly right (IMHO)! The main benefit to a second battery is you will not get stranded from a dead battery. Just move a selector switch from A to B and presto you have a good battery to start your engine and get you home (assuming dead alternator got you into this predicament or playing the stereo too long).

I am going to assume you have a volt meter in a '92 SNOB. So, what does it read with all accessories turned on after an hour or so of boating? Remember that the volt meter basically justs tells you the condition of your battery or how many volts the battery is putting out. With a healthy alternator, you should be reading about 13.5-14 volts at all times. If the reading starts out in this range (fully charged battery) and then drops below 13.5 volts or so, then you are drawing more current than the alternator can supply and you are discharging your battery.

If you had an AMP meter, this would be easier to figure out as the meter would be below center if the current draw was higher than the alternator could put out.

So, to me, AMP meters tell you real time status of your charging system. However, the drawback is that it requires a heavy guage wire to the meter which is directly connected and inline (wired in series) with your charging system. In other words, a weak link that can cause electrical fires. Conversly, volt meters just measure the condition of the battery and only require a small wire (wired in parrallel).

Sorry got wordy...what were you asking? Oh yea...you probably just got a battery that crapped out quickly. Remember that deep cycling "starting" batteries shortens their life span significantly.