This ink is waterproof though it may not seem so at first glance.
This ink will smudge when it first encounters water, but after the surface ink that did no fully bind with the paper washes away, you are left with very black, permanent writing. How much surface ink exists seems to depend on the paper. In Molskine notebooks it smudges a lot, but unlike the Aurora and Jentle inks, the writing remains legible. It smudges less in my police notebooks.
Kiwa-Guro also makes my extra-fine nibs glide across the page as nicely as the silky Aurora ink. This ink will be another ink I will use for a decade or more.