Dukhan is about 100 km (correct me if I'm wrong) from Doha...take the new road, it's unter construction but already good and ver smooth, much better than the old one. You go to education city and then just follow the signs to Dukhan.
The water is nice, but I did not find the white sand I was dreaming of, many many shells and not very fine sand - maybe I was at the wrong spot?