5 kWh/(d.m2) summer maximum absorbed insolation by a PV panel, or some 18 MJ. 

10 cm of water is 100 kg/m2, needs 420 kJ to warm by 1 K. 

So, sun can warm it by 43 K, if no losses are present and if a PV layer 
would float on it.

A concrete cube with a 15cm side would be warmed just by some 15 K, if 
white and covered with a 1dm2 PV. Air-cooled, this should drop to below 
10 K, close to the air temperature span.

July 13, 11:12 plexiglass put off, PV is bare glass
         21, put onto a white concrete cube, with a 20cm side