I've been putting tap water on the same Polar HRM chest strap since 2004 and it works flawlessly. I get interference when there are lots of telephone wires overhead or when an ambulance passes but that's a rare occurrence.

If I put it on and don't get instant readings the fix is always to tighten the strap a bit. Along those lines, a few years ago I replaced the elastic as it had worn out and wouldn't stay tight. With new elastic, I'm back to consistent readings every time.