My sump pit is pretty shallow, and is dry about half of the time. My challenge is that the sump is on a float switch which doesn't activate until the water level is just about an inch (15mm) below the bottom of my slab. It doesn't take much of a delay for it to overflow.

There are three sensor types I'm aware of: a resistive "dipstick," ultrasonic distance gauge, and a bathymetric depth gauge.

The resistive dipsticks are only about 6in (15cm) in length, and would be good to determine if the water is within that distance of a target water line. I would position it on the side of the pit with the effective upper end set some buffer below the top surface of the slab floor, and possible gram the logic to alert when the water lever rises above the bottom of the slab. I would otherwise be blind to the water level if it is below Tue bottom of to sensor.

The ultrasonic sensor looked interesting, but I have pets and I try to avoid ultrasonics in the environment when I can. It also strikes me as perhaps the least reliable (or most crosstalk-prone) of the sensors because it has a larger field of view, and might be fooled by the equipment in the pit.

That leaves the bathy sensor. Its basically a diaphragm and a pressure sensor you screw to a waterproof housing, and submerge. Placed near the bottom of the pit, it would read the depth of the water by the pressure of the column above it. Most accurate, but also the most expensive. Last I checked, the sensor itself was US$60 on the usual online markets.

Ultimately I'll probably do a length of copper pipe with a 90 elbow and a cap. I'll drill and tap a port through the cap and thread and seal the bathy sensor into it, and feed the wire back up the pipe to the controller. I'm not sure if there a driver for it in ESPHome, but if nothing else I expect I can probably just read the analog voltage and alert on a set point.

I haven't really fleshed it out as a design yet, but that's what I'm most likely to do.