I was recently able to get this project working with a UFM01 ultrasonic water flow sensor from ScioSense https://www.sciosense.com/ufm-01-ultras ... ng-module/ (available from Digikey and Mouser for about $37) and an old Metro ESP32-S2 I had laying around. There is a recent Arduino library for the sensor, but I wanted to try it with Circuitpython. The ultrasonic sensor was a huge improvement over the Hall Effect pulse counter sensors I had been using, as they would frequently clog, and Circuitpython isn't as good with interrupts as Arduino.
The sensor itself communicates over UART to a hardware Serial port of the ESP32 (I chose IO5 and 6, as it lined up with my existing Arduino R4 shield). It sends out a BCD hex string with the serial number, flow volume, rate, and -bonus- water temperature, which can be parsed out and displayed on the serial REPL.
Once that was working, it was on to a WiFi enabled interface. The sensors are located away from any wireless networks, so the ESP is set to Access Point (AP) mode, and the user can connect to it as one would any other wireless network (192.168.4.1 and password). Security was not a major factor, so the password was kept simple. The measured parameters are displayed on a web page to either a laptop or cell phone, as well as the max flow rate (the water is pumped through the sensor, so I use it to gauge the pump health). It took a bit to get a reset button working, but the button now resets the accumulated flow logged in the sensor. (the total flow volume remains stored in the sensor's memory after removing power).
I know the styling of the code does not use the standard settings.toml file, but can be modified if needed. The attached code.py and ufm01-circuitpython.py files are all that are needed beyond the stated includes from the adafruit bundle. It is currently working on the ESP32-S2 Metro board; you can try other WiFi enabled boards and tell me how that works.
This is a project ported from the Arduino platform, and has the advantages of much faster changes than re-compiling and downloading, especially if you just need to make minor changes. The Arduino project had the code derive a unique SSID based on the ESP32's MAC address, but I kept it simple here. Most of the code length is for the web page display and controls.

Technical notes:   The nature of the ultrasonic sensor is that the elements need to be immersed in water. The sensor will throw an error code if no water is present. Be sure to place is in a way that it stays full of water when any is present, like a P-trap or similar. I have mine placed above a pump's check valve.
ScioSense has a good writeup in their data sheet online. I was very happy to see an affordable and easy to implement ultrasonic flow sensor, as the turbine types I had been using would clog constantly due to small debris in the pump discharge.
The only issues I have had with these sensors are: 1: There is no reverse voltage protection diode, so either add one or be very careful when hooking it up. 2: The point where the wires enter the potting compound on the sensor is weather proof but not water proof. If the sensor is immersed in more than about a foot of water, water will eventually seep in and render the sensor inoperable.
There is also a 1-wire version of the library that uses the brown wire (seen unused in the picture). When I was working with the factory tech support (before the Arduino library came out), it was not yet supported. It now seems to work, with Arduino support for ESP32 and Mega 2560 platforms. I am testing it with the Arduino R4 Wifi, and have had some issues, which is why I gave the Circuitpython a try.

The first python code is a driver that parses the hex BCD output from the sensor.  It is called ufm01_circuitpython.  The second code has the webserver, etc.  This is my first attempt at posting on adafruit-playground.com, so bear with me, and moderators/admin- please feel free to edit this for style and content.