I would use either:
1) Microfiber cloth moistened (not wet) with Simple Green. It works better than water getting key gunk off, which is mainly oil, dirt and dead skin.
2) Magic Eraser. These work great, but they work great because they are a gentle abrasive. Keep this in mind and be gentle to avoid damaging the printed letters on the keys. One other thing...don't use these on HP keyboards. HP keyboards, at least the cheaper ones, use little decals that aren't very durable. Magic Eraser works fine on MacBook keyboards without causing damage to the letters, not so much on some others.