Yup, I have added distilled water to mine (just enough to cover the plates) and I am certain that it helped extend the life of the battery. I just replaced it last summer (the "eye" was red).
Be careful (i.e.--where safety glasses) when adding water to the battery because it can burp back out (into your face!). Keep the garden hose handy to rinse down any acid that might spill out, too.
Yeah, just pry the caps off with a large screwdriver and *ass*...er add water until the level hits the bottom of the plastic holes. Then pound the caps back on. Done.