You need to be specific. How much water? how is it to be contained? How quickly must it be heated?
Eg 100mL of water in an insulated beaker to be heated from 15C to 90C in 6 minutes.
What is your power supply?
The power is V^2 /R so you choose a siuitable fixed resisitor and control the voltage.
Or
(this is clever)
you can use a pair of power transistors to make a voltage controlled active load.