Inspect the underwater ground surface of ponds at the following locations:
[4683,14633] (south of Horni Vltavice)
[14708,12888] (Zbytiny)
[4278,1250] (Wüstfeld)
[12785,13947] (west of Blazejovice)
[5328,9259] (Ceske Zleby)
[10742,10282] (Volary military base assault course)
[10971,11870] (farm north of Volary)
[7133,7715] (Stozec)
There may be others, but these are the ones I've found so far.