I would avoid water also, if there is any ‘printing’ on the non-play surface. Often it’s a paper based label and water and paper don’t react well.
I’ve been cleaning CDs and DVDs for a number of years (mostly rental DVDs) that have become so dirty that they don’t play (or play poorly).
I found that the best thing is isopropyl (rubbing) alcohol. It’s best to squirt it on or use an eye dropper and let the alcohol soak into the dirt for 30 seconds or so.
Then I remove the alcohol with a soft cloth (I get the free ones form my eye doctor) by wiping lightly. I often read that one should not use a circular motion when wiping the surface, but that has never been a problem. Just don’t rub hard. Rub hard and you end up with a clean disc that is scratched.
If dissolving the ‘crud’ with acohol and wiping lightly (almost like polishing) doesn’t clean the surface you probably need to go to a DVD store that offers re-surfacing (about $3 per DVD).
When the surface is clean but [U]scratched[/U] to the point that you get mis-reads, I’ve saved a few DVDs with liquid furniture polish, maybe 20% of the time.