The Achensee (Achen Lake) contains about 6 dive sites that include a parcours and some WW2 material. The dives are described in German on the website of diving hotel ‘Post am see’

http://postamsee.at/de/region-achensee-sport-wassersport/tauchen-am-achensee

The deepest part of the lake is about 180 meters, but most of the sights for diving are in between 12 and 30 meters. Some of them are marked with surface buoys. The lake has a rocky bottom and spearing is not allowed. If you are not going to dive at the dive sites bring a surface marker or you might get hit by a sailing boat.

Here is a freediving movie from youtube by Alpnoemanni. It has a low frame rate but shows good visibility:

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