i would drain the whole thing, using 10-20lbs of baking soda to nutralize the acid, then buy a gallon of acid and refill it.
have done it before during a spill.
usually any time a cell hydrometer reading is less then the others, that cell is shot, and it makes the whole battery bad. usually the float is VERY similar, you can tell if one ball is about to float, or about to sink, so its quite analog even though its 5 ball digital.
IF it does have water in it from rain, i cant think how that would happen, then its probably recoverable.
it does not sound to promising, and it was weird that it was so well charged, mabey it was not so well charged, but that the "specific gravity" reding was effected by dilution with the water.
i cant remember what happens with the hydrometer in water, but YOU could check, test it in water, if all the balls FLOAT, then mabey they were floating just because the acid is DILUTE, not charged.
just more thought.