Weird as I am, I can't tell you exactly how much water. I can tell you this, though.... I used filtered water through a Brita pitcher.
Usually, I'd pour about 2 fingers of the Lagavulin in an "old fashioned" glass and put in maybe a teaspoon or so of water. It wasn't a "science" thing though... I just put a splash in and gave it a swirl and let it sit a minute or two. It really made the "peaty" taste more subtle and just great. If it tasted a bit to watery... easy to fix. If it needed a bit more water... a quick flip of the wrist and it was done.
Since that experiment, I've taken to adding small amounts of water to Scotch. I do like the result.
Like Habu, I've seen the "add water" comment from folks that are supposed to "know" about Scotch too.
For those that want an "official" opinion:
Ask the Scotch Doc