I would turn water poisoning from a random and deadly inconvenience into something that has to be maintained by bathing or taking cover. Long story short, you have a meter. If that meter gets too high, you get water poisoning, which starts out as a weak DoT effect (1 or 2 per tick) that grows much stronger over time, eventually being lethal. The meter increases faster and decreases slower as you go deeper into the dark sea. Soap and bathing will always clear the meter instantly, but standing under cover will slowly but steadily reduce it until it’s gone.
Additionally, when water poisoning gets to a lethal level, the meter will begin flashing red, and hovering over it will explain how to not die