I've often been amazed at what you can get away with when it comes to lead acid batteries. At the same time, they are sometimes destroyed by the simplest of things. It can seem almost random at times.
As others have mentioned, charge them up good, run a desulfator on them if possible, and see what happens. You might also want to check out your charging system as you shouldn't have to add water very often if they aren't overcharging. Also, if you do purchase new batteries and they are AGM type remember that AGM are much more susceptible to damage from overcharging so you'll want to check your charging system out well no matter how things work out with your batts.