I was going to add my knowledge about Squaw Creek, but most of all the facts have already been laid out for broncrider. I have been there several times. When I was there in the late 90's and fishing, the water was not particularly clear enough to have much vis for diving. As it is a cooling lake for Commanche Peak nuclear power plant, the water does get quite warm, as mentioned above.
The times I've been there, the water temps were posted, and were near or well over 100 degress - in the summer months.
Also, my mother, sister and step-father went there to attempt dives for their open water certification, also in the late 90's, and vis was very, very poor.
I also once knew someone who worked for the security company contracted to provide security to the plant, prior to 9/11, and even then security was very strict - I doubt it will ever open again to the general public.
Suprisingly, after 9/11, and a year ago or so when I was there, one is still able to access the downstream side of the dam at Possum Kingdom Lake. You can walk along the security fence, on the river side, or even enter the water, and get very close to the dam. Here, and at the bridge further downstream along Hwy 16, is a good place for trout, striped bass and catfish - for those who like to fish...