Floating swimming pool thermometer made by Astralpool.
 
A pool thermometer helps monitor the water temperature to ensure it’s comfortable for swimmers. Temperatures that are too cold or too warm can make swimming unpleasant. Generally, a comfortable swimming temperature is between 78°F and 82°F (25°C to 28°C).
 
Water temperature affects the balance of pool chemicals. For example, warmer water can accelerate the evaporation of chemicals like chlorine, while cooler water may reduce their effectiveness. A thermometer helps keep track of water temperature and adjust chemical levels accordingly.