This idea is to add a new potion effect, water vulnerability, which causes mobs or players who are under its influence to take damage from touching water, and perhaps from touching rain. Mobs under its influence might change their pathfinding to avoid water or rain.
The potion is brewed by adding an ender pearl to an awkward potion in a brewing stand. All of the typical variants (splash, lingering, strengthened, extended) would be available. The default duration might be about a minute.
A Suspicious Stew can be crafted which gives the effect, using a bowl, a red and brown mushroom, and a chorus flower.