Or if you occasionally spill coffee or any other liquid ever. I sometimes hand-write recipes. Fountain pens + sloppy kitchen meal prep are a bad combo.
You can get fountain pen inks that chemically react with cellulose for complete water resistance (there may still be a little smudging from unreacted ink washing off the paper). Noodler's Black is a famous example. I like this kind of ink, although it's not without its drawbacks, because cotton/rayon/other plant fiber clothing is also mostly cellulose and will be stained just as permanently.
I've used only Noodler's Bulletproof Black for years (in a TWSBI Eco piston-fill) - it works great, and I realized if I was going to take the time to write extensive notes it was worth having some protection against water.
I previously used Waterman Black - it flows really smoothly, but it isn't waterproof. Also, it doesn't seem quite as dark as the Noodler's.
Platinum Carbon Black is a wonderful ink. It seems to work very well in cheap fountain pens with flow issues. It’s highly resistant to coffee spills and looks ok on mediocre paper. Only drawback is cleaning it up; it cleans up like used motor oil.