The large body of water is Srah Srang, which was either built for irrigation purposes or, more likely, religious ceremonies. The platform in the photos was the king's dock, and is now occupied by begging kids. One downside of Siam Reap is there are dozens of little kids following you around everywhere, selling postcards, drinks or just begging. They're cute and friendly rather than aggressive, but they are persistent so you either have to buy lots of postcards (which of course attracts other postcard vendors notwithstanding that you just satisfied your postcard needs with the previous vendor) or tune them out. I've also thrown in a photo of another temple, Prasat Kravan, at no additional cost to you!