Is it practical to boil drinking water in a pressure cooker?
Overall it will make little if any difference. 

The pressure cooker won't reach 100&degC; noticeably quicker than a normal pan with a close fitting lid, and the pressure cooker is made of thicker metal which will take more energy to heat. Simply closing the lid would help quite a bit. 

If you want to hold it at a boil the sealed lid of a pressure cooker would help, except you can't really monitor the temperature. Waiting until it whistles will mean it has been boiling for long enough to produce enough pressure for a whistle, but that's still a measure of pressure, not time. 

There are a few species killed by the temperatures achieved in a pressure cooker, and not at 100&degC; but my understanding is that these aren't the pathogens that tend to contaminate drinking water.

As you're flavouring the water as well, you may have to adapt the quantities of seeds if you use a pressure cooker, because the flavour will extract faster.