I was issued one of those tactical clicker write in the rain pens. I am usually wary of anything hyped up as being "tactical" but it's solid metal, dosen't freeze or smudge, I nice to write with,and the dammed ink lasts forever. I have since purchased several of my own I like them so much.

I haven't come across a situation where I had to write upsidedown or underwater yet, but who knows.