Using a capacitive sensor is possible to “detect” the presence of water within a tank attaching it at a fixed height, so when the water goes down this minimum level the sensor will trigger some alert.

For this purpose I’ve used a XKC-Y25-NPN sensor and a Wemos D1 ESP8266 module plus some few ESPHome code lines.
The wiring requires just three connectors, pay attention to the sensor wires colors, different suppliers could change the order of them:

3D printed case available here
ESPHome code is the following:
esphome:
name: plants-water-tank-sensor
platform: ESP8266
board: nodemcuv2
# Enable logging
logger:
# Enable Home Assistant API
api:
ota:
password: "670d70c6dd4ea29b3b9b66b1109dc0d8"
wifi:
ssid: !secret wifi_ssid
password: !secret wifi_password
binary_sensor:
- platform: gpio
pin:
number: D2
mode: INPUT_PULLUP
name: "Plants Water Tank Low Level"
filters:
- delayed_on: 10ms
The binary sensor will be available on ESPHome Integration

and usable on any dashboard

or automation:
alias: Allarme acqua irrigazione
description: ''
trigger:
- type: turned_on
platform: device
device_id: 107cdcec7a2e43bb990398295828afa5
entity_id: binary_sensor.plants_water_tank_low_level
domain: binary_sensor
for:
hours: 0
minutes: 0
seconds: 2
condition: []
action:
- service: notify.mobile_app_sm_a525f
data:
message: Low Level Water
mode: single