I only have had one ink, in the past, that was bought specifically for these characteristics, and that is Ecclesiastical Stationery Supplies Registrar's Ink. It is an IG ink made in England, and I have enjoyed using it for occasional writing (watching the transform from dark blue to black is intriguing) as well as having it in a dedicated pen for addressing envelopes. I could certainly live without it, but one bottle has lasted me 2 years already, and will go for a few more.
I recently decided to pick up a non-IG ink that would be as water resistant/proof as possible. Why? Because I wanted to start writing postcards. Nothing more Earth-shaking than that, but I thought it would be fun to have another dedicated setup. I was all set to get a bottle of Noodler's Black, a long-time standard, when Sam at Pendemonium sent out a note about their exclusive ink, Legal Lapis (also by Noodlers) that happened to have very good qualities. I like them, so I bought a bottle. It turns out just a tad greener and lighter than I normally like, but a few drops of Quink Permanent Black toned it down.
I have now used it in a true Frankenpen created just for this project: a small Sheaffer Feathertouch nib from a trashed Balance installed in a hilarious and garish Wearever body, the near-extra fine nib putting down just the perfect small script to allow a few words on the postcard. It makes jotting off a few cards to friends a fun and relaxing time, and with this ink I don't have to worry about the mail (on their end, not here in SoCal) getting wet.
And that's my stash: 2 bottles, which will more than likely last years at this rate.