Due to issues related to a chronic cough I have, I have lately been doing sinus rinses with saline water. In principle this is exactly the same as a rectally inserted enema- you are putting water into a bodily activity. You are just squirting it up your nostrils rather than up your rectum.
The instructions that come with the sinus rinse, as well as the medical advice I received from my ENT doctor, was to ONLY use bottled, distilled or purified water when mixing it with the salt packet. I would think it would be particularly dangerous if one of those flesh eating amoebas were in the tap water and I shot that up my nose. I would probably be all done. Can you imagine having something eating your brain?