Here is what I know about Round Valley from trying to dive there and reading and asking questions. I got most of my info from the Whitehouse Aquatics shop owner, located about 40 minutes from DS. First, the Whitehouse Aquatics dive shop owner (I forget his name, it's in my log) says he built the platform in there about 15-20 yrs ago. It's in 20-30 feet of water and right now the temp is high 70's down to about 30 feet and then it gets very cold with a few thermoclines down to 165 ft. As for the city, he said that nothing exists now but some small rubble piles and walls. He showed me a picture of the place being torn down and bulldozed before they flooded it. It looked pretty flat and sandy to me. Also, he says that it is pretty much just a large mud flat with nasty silting possibilities and very poor viz down deep. He also said that what they didn't bulldoze was finished off by the National Gaurd using the former town for bombing and shooting practice. If you call the shop, he will answer questions in a grumpy tone but friendly manner.
I have never been diving there but have hiked and waded in the warm and clear water. It looks great in the shallows and the three fish are suppossed to be quite tame. I do plan on diving shallow there soon. Maybe on Sunday.
One other problem is that diving is only permitted (by the Ranger) in a small diving area next to the shore. You may run into a problem with a surface swim out to the center. But what they don't see may not hurt them...
As Xjae mentioned, I tried to dive there last week but the new "out of the water by two" rule (I believe that goes for weekends too) went into effect after Labor Day so we went to DS as it was already 1pm when we got to the shop.
Someone posted the link to the Scubadiving.com report above. That report is somewhat helpful but still the leaves a mystery over the whole town part. Well, I saw the photo. It's not worth diving it deep but that's just my opinion. Nonetheless, be safe and make sure to do a report as we are all still interested in knowing for sure. Be safe!
Jason