Attached is a very basic thermistor circuit that should do pretty much what you want it to do.
However, there are a few caveats to all of this. First and most important is you mention a 55 watt load for your heater (lamp) and you mention 6 Volts as a battery source of power. A 55 watt lamp running at 6 volts will draw 55 watts / 6 volts = 9.17 amps of current. Unless you have a very, very large 6 volt battery which would be very heavy your battery will have a very, very short life. Next, you never mentioned the volume of water or container type or exactly how this is to be configured. If I place a calculated heating element in a cup of water I can likely raise the water temperature. If I place that same element in a lake it won't have any effect. Using a lamp to heat water as you mention likely will be a very inefficient way of going about this.
The attached circuit is a basic on/off controller. The relay it drives would need to be slaved to a much larger relay capable of handling the heater load current. RY 1 needs to have a coil voltage of 5 volts that draws less than 150 mA so transistor Q1 can drive it. Again, the circuit is an overview of a basic on/off heater control.
Ron