unless youve rolled upside down and havent noticed?...
then possibly your alternator is overcharging and boiling the battery. It dosent still work does it?- was it like bone dry?
when filling you want to just cover the plates by around 1cm usually. some batterys have a plastic 'foot' which goes down from the breather to show the level to fill to. others are clear and have a level.
have you noticed any smells while driveing?- boiling batteries sometimes smell like rotorua