TFA Dostmann Analog Swimming Pool and Pond Thermometer, 40.2004, with fastening cord, measures underwater, for whirlpools, 

Information about this article:

• Installation : Pool thermometer with fastening cord; device remains near the edge.
  
  
• Features : Convenient reading of the water temperature in Celsius
  
  
• Measuring device: Thermometer measures from -15 to +50 degrees Celsius, large temperature display
  
  
• Use : Ideal as a thermometer for pools, swimming pools, ponds, aquariums, hot tubs, ice swimming
  
  
• Scope of delivery : The package includes the analog thermometer.