Thermistor temperature dispersion
It is going to be difficult to calculate the voltage/wattage necessary for this.  I suppose it could be done, but why not do it by the "seat of your pants"?

My unscientific opinion says you would need on the order of 100+watts to raise the temperature to 100C at any miniscule water flow, and you probably won't find a thermistor rated anywhere close.

I propose to adjust your method as follows.  Use two thermistors, one set up with very low self heating as a reference (placed upstream), and one similar to what you propose with 33 ohms, which at 10V would equal 0.75W, which should be enough self heating to do what you want.  Be sure to calculate all temp ranges (including accidental open air possibility)so you don't go over the watt rating of your chosen thermistor.

You may be better off using a PTC for this application, it could self limit current.