I use a Pure Freedom 5KW diesel heater.
On max the temperature fluctuates between around 53c and 65c.
I can't make it run any hotter even by recirculating the water into the tank to raise the ambient temperature of the water in the tank, the heater just adjusts it's output to keep the temperature between these parameters.
I'm happy with the temperature at the brush up to around 40 metres, but then it starts to cool down too much, especially on chilly days.
This means it's all about the hose.