You could use a door sensor.
Here are some pictures (only german): Z-Wave Binärschalter selbst bauen
You can solder wires at the magnetic sensor and these wires can be positioned at your max. water level. If the water reches the wires, the sensor will alert Homey and you can do some stuff with flows.
It’s possible to ues Fibaro (Z-Wave) or Aqara (Zigbee) sensors. The Aqara sensor offers a device option to invert the alarm signal. Then you get an alert if the water connects the two wires.
This way I built a rain sensor (Aqara door sensor and the rain sensor circuit board). Pictures for it: Suche Regensensor und Warnung erstellen, wenn Fenster noch offen
