Heat transfer of a water body like fish pond and lake

As an introduction, I am a computer engineering student that want to create an arduino project in which it use water temperature as its parameter. The temperature will help the microcontroller choose whether it should turn on the heater or not. But in planning this project, I cannot decide how many heater and how many temperature sensor I should put. Do I need to measure the temperature in different spots of water because temperature difference in each spot? How the heat will propagate from one spot to another? Will there be an area in which the heat isn’t spread enough from the heater so I need an extra heater in that spot? The body of water that I’m using is relatively calm like a pond and lake.