If you have to top off the cells, it's OK to use use clean tap water unless your local municipal water system has an extreme amount of dissolved minerals. Slow-charge the battery overnight before checking specific gravity or you might get a false reading from the water you just added not mixing with the acid thoroughly. Full-charge specific gravity should be somewhere around 1.265-1.300 on all cells, with no more than a .015-.020 variation between cells. A totally discharged cell will have a S.G. of 1.100 or less.
Electrolyte that gets below the level of the plates for an extended period of time will allow the exposed plate material to develop "sulfation", an extremely hard deposit of Lead Sulfate that is very difficult to dissolve by recharging the battery. I have had some degree of success reclaiming a sulfated battery by repeatedly discharging and slowly recharging it, maybe 10-15 times. Use an old sealed beam headlight to completely discharge the battery, then recharge at a 10-15 amp rate. Repeat as necessary. Good luck!
Jerry
