My entire bullet journal got soaked at the community pool last summer, and now I only use waterproof pens.
I was trying to plan my week while my kids swam, and a rogue splash from a cannonball soaked the whole thing, blurring two months of spreads into an unreadable mess.
Waterproof paper is a smart move, Jade. The real problem is that most pens use water-based ink, which is why they run. You need a permanent marker or a ballpoint with oil-based ink to actually beat the rain.