Waterproof+1

No Pal spawns with Waterproof as a guaranteed innate passive. It appears as a random passive when catching wild Pals, and it's a standard-weight roll so it shows up fairly often.

The most reliable path is selective breeding: if either parent has Waterproof, the offspring has a chance to inherit it. Stack it across generations by always using a parent that carries it — each child can inherit up to four passives total.