What does this switch do ?

Yes I think it is manually operated without a thermostat because the temperature today and the last couple of days has been around 20’C and the temperature of the water in the fresh water tank which is 25% full would be a bit less.   

So if it was designed to cut out via a thermostat at a water temperature of say around 5’C surely it would not come on today ?  I can see that the heater is coming on because of an increase of 2.5A drain on the battery.