[dieantwoord]

I had such a space pen about 20 years ago and it was already possible to write with it under water: https://en.wikipedia.org/wiki/Space_Pen

[iancmceachern]

Pencils also work well, there are pencils specifically designed for it.

https://www.xit404.com/collections/aqua-pencil-solo-packs

It's what scuba folks use.