Usually exposed plates is a death certificate for a battery. The exposed plates usually never contribute to the capacity of the battery from that point.
Water usage is directly realted to how much time they spend at gassing voltages when charging which are generally anything above 14.1v, but this is not written in stone for every battery.
Unless the DC loads are extreme, there is no gassing during discharge and thus no water loss.
Batteries will increase their water consumption when aged, even if the user does not increase absorption voltage or duration, and the duration required on an Aged battery also increases, furthering water loss in addition to simple age, if the user was wise enough to increase absorption voltage duration, if their charging source allows such things.
I am glad your batteries appear to be OK, but they have likely lost a good portion of their remaining capacity, and will decline faster from here on out.
Not to say the time they have left will not be useful, or Subjectively 'just fine' for X amount of time, but I would recommend keeping that ~200$ buffer handy for if one of the cells shorts out and you are left with a 10.5v battery bank that is dangerous to charge as the shorted cell withh heat excessively and gass a lot.
Earlier this decade, I prematurely killed 2 pairs of paralleled marine group 27's by not checking thier water levels in time. This was mostly due to the very inconvenient location in which to check their levels, but also ignorance that their water usage increases with Age, and not linearly either. It seems from half life onwards monthly checks are required and a surprising amount of water is required. I hated having to go out just for for Distilled h20 to add to them but tap water will speed their decline.
One set that was low on water, became useless instantly, the other lasted for 8 more months worth of frustrating perfomrance and loss of confidence. They were only exposing about 1/2 inch of the plates as well so it is not as if Half of the plates were exposed.
6v GC batteriess are way more tolerant of such abuse, but i would recommend applying an equalization charge to them and try to get specific gravity back upto 1.275. I'd be surprised if a hydrometer reading on your lowest cells would read above 1.200 right now. Resting voltage will mean so very little at this point please take no reassurance that seeing 12.6v plus means much, unless they still hold 12.6v+ in the morning after using them normally all night long taking more than 40Ah from them.
An EQ charge soon could likely extend their remaining lifespan a fair amount, but there is no guarantee it will.
The brief explanation of an EQ chrge is after the normal absorption cycle is complete, meaning a normal 'full' charge, raise battery voltage to as high as 16.2 volts, and hold it until Specific gravity no longer rises, or battery temperature starts rapidly rising.
The EQ charge performed regularly or when needed can greatly extend flooded battery longevity, but often when performed, is much too little too late, almost like changing the oil on an engine that just seized due to lack of oil..
Getting to 16.2v can pose issues too. Few plug in charging sources can do so. I used to do so by changing my absorption and float settings Mid day when the batteries were already full and my 10 amps was enought to get battery to as high as 16v. Often it took 4 hours at 16v, to max out SG readings on my OTC4619 hydrometer. Remove all loads from battery when 15.5v is intentionally exceeded.
How much damage was done cannot be definitively answered without doing a 20 hour capacity test, and doing one of these accurately is not easy to accomplish as it requires a constant amp load as voltage decreases, and a battery temperature of 77F for the 20 hours at the prescribed constant load.
A regular load tester will show little, despite the misconceptions that they do. Load testers are to show if a starting battery has enough ability left to crank a motor, and are of limited usefulness when applied to used and abused deep cycle batteries.
