Yeah I wish there was more of this happening. BTW what happens when your water is fully hot and the sun is shining - does that panel detect there is no load so doesn't generate?
I have a mixing valve on the output of the water heater to mix with cold down to about 50C. This lets you get around 2x the nameplate capacity of hot water out of the tank so there is enough to bridge a cloudy day.
To get the full capacity of the panels, it necessary to choose a water heater element that matches the source impedance(Vmpp/Immp) of the solar panel array.
Obviously you'd want a cut off at some temp though, for safety concerns.