Severely discharged battery...does it need to be replaced?

[hellosailor]

"and still showed 13 volts afterwards."
You might want to double check your voltmeter. If this is an analog meter installed in the boat, they're often not very accurate. With an alternator running, it should show 14.3-14.4. At least 13.8 even at idle with a cheap regulator.
But then, when the alternator is off, a "good" battery typically will show 13.6 volts for a short time, which is 12.6 plus another volt of "surface" charge. 13 itself...is to ohigh for a battery without a surface charge, too low for a good battery with a surface charge.
Odds are that you're OK, but get a reality check on the voltmeter. With a conventional wet cell battery, a hydrometer is a good way to do that. Spring for the one the size of a turkey baster--not the little six incher.

[Kiltmadoc]

How's this for abuse: We have a deep cycle/start battery that (surprisingly) still shows some voltage after losing about 2/3 of a gallon of distilled water. The previous owner didnt check the battery, and neither did we until I had to get a jumpstart from towboat US ($185!!!!!).

I got one new battery so far, but the old one that was missing the water ( i replaced the missing water) seems to be draining its voltage very quickly.

Here's my question: I now have one new start/deep cycle battery, should I get an AGM or some other type of battery for the 2nd battery when it needs replacement?

[TQA]

Originally Posted by Kiltmadoc

Here's my question: I now have one new start/deep cycle battery, should I get an AGM or some other type of battery for the 2nd battery when it needs replacement?

NO Assuming you charge the batteries in // then get the same type eg a flooded wet cell.

AGM batteries need a different charging regime.

[hellosailor]

"I now have one new start/deep cycle battery, should I get an AGM or some other type of battery for the 2nd battery when it needs replacement? "
Ideally batteries in one bank should be "same same" even to being from the same production lot. So I'd replace the bum one now, rather than later, and match whatever your other battery is. If the bum one still has some life in it, use that as a separate starting battery or alternate bank, till it quits.