Hi,

I hope it's not too late to answer! I dived several lakes in Austria, and I can say they have some really nice ones. Maybe the most special place is the Gruner See, which can be dived between May and August, and famous about the wonderful, clear water. Visibility is up to 40 meters!
Erlaufsee is good place to meet pikes and other fish. I had good experiences in Traunfall, Klopeinersee, Attersee as well. Most of the lakes are free to dive, with several diving spots, you can find parking place, toilets etc. There are many dive centers where you can rent anything you want.
When to go? The lakes in the mountains are cold under 10 meters in any time of the year, but I would say from spring to autumn most of the lakes are OK for diving. I suggest drysuit or thick wetsuit all year. In the winter diving is more complicated, many of the lakes are frozen and ice diving has special rules.
This is a really good website about Austrian lakes (in German):
http://www.taucher.net/tauchfuehrer_...ebersicht.html

And here is a video about Gruner See diving, shot by me.

Hope I was able to help.

Rudolf