Float-level sensors come in a wide variety of styles, materials, and sizes.

I was thinking there must be a simple sensor that could tell me when the water level has fallen a set amount and automatically run the "fill pool" zone on the Rachio.

The sensor straddles the wall of the 1.7-liter water tank, so no penetrations are needed. Inside the tanks is a track that guides a copper and PETG float that’s sealed with food-safe epoxy resin.