It doesn't need to be variable. Virtually all heaters, including the heater/furnace in your house, or your oven, work by cycling on & off. Power is turned-on until you hit the target temperature, then it's turned off. And on again when the temperature drops. Refrigeration and air conditioning works the similarly.
Usually, there is some hysteresis, so for example, if your target is 200 degrees F it might turn-on at 199 degrees and stay on until 201 degrees. That prevents the relay from "chattering" or the thing turning on & off several times per second.
It's also possible to "dim" the power with PWM or a circuit like a light dimmer, which works similarly to PWM but it "chops" the AC waveform. But an on/off relay is easier to use and it actually works better for temperature control.
Of course, that means you have to monitor the temperature.
You just have to calculate-estimated the required minimum power (wattage), calculate the resistor value, and then select a power resistor rated for that wattage (or more). Some safety margin is recommended.
To keep it safe, you should use low voltage (less than 50V) and that will mean a "big" power supply. Even at low voltage, it should be insulated (with heat-shrink, etc.).
Or, you can buy a submersible water heater that's designed to be run from the power line (switched through a relay). Then of course, the power line is the power supply.