We've got a small pond in our garden, but with the warmer weather in summer I will often need to top it up manually with water when the level gets too low. I thought it might make for an interesting project to have a battery powered IoT device (likely ESP8266 based) track the water temperature and water level. I've found a waterproof temperature probe that should be fine for temperature measurements, but I'm looking for suggestions on measuring the water level (the pond is around 50cm deep). There are plenty of float sensors, but they only appear to give high or low sensing (which isn't ideal). What would your suggestions be for a low-power, low-cost water level sensor be? Would a submerged pressure sensor be the best way, waterproof ultrasonic sensor, or are there alternative float sensors that would enable me to take level readings?