I've done a ton of cold water shaving without a mirrior but always due to being left without other options. Heck, we just called it "Field Shaving," ... and its doable but you don't get nearly the same shave no matter how much practice you get. And I must say, I've never and will never do this with a straight. All of my field shaves were done with a Mach III and gillette shaving goop.