Nothing, and I mean Nothing which is not sterile should have anything to do with your bladder. Period.
Yes tap water will work. And most likely you will not get an infection. Most times. But it only takes once. Trust me - I've done it to myself. You do NOT want to go there. It's just asking for trouble. I played with this before I knew about sterility and the dangers of this (LONG before I cam across this site). And for the first month, everything was fine...but then one day...
For those of you who love Russian Roulette...sure - slam yourselves with random tap water. For those of you who wish to be safe and have a long, healthy career in play, well, don't fool around with this stuff.
Bladder irrigation should be done with sterile water or saline (in this case it doesn't matter I don't believe). The balloon on a foley catheter should be inflated with sterile saline or water. Sterile procedure should be used during insertion, removal, or interaction with the catheter. Period. It's not optional.