[freosam]

I've found it depends on how soon you want it to be waterproof: I've had ink run when it's got wet within a few weeks of being written, but the exact same ink on the exact same paper (Watermans in a Moleskine) has not smudged a bit when getting damp two years after being written.

Not that there aren't inks that are liable to be much better than that, sooner. But really, if an ink is to flow through the pen, and not have it clog things, I reckon it's unlikely to ever be able to be completely waterproof right after being put on the paper. Unlike a 10¢ ballpoint pen with oil-based ink! :(