Temperature expresses how hot or cold the water is Or more technically the kinetic energy of the atoms and molecules
Heat energy from sunlight, air, and thermal pollution influence water temperature
Water temperature influences other parameters, cold water holds more oxygen and hot water is less dense than cold water
Water temperature can be measured with a thermistor based temperature probe
The thermistor is embedded in a thin-walled tube and encapsulated with a thermally conductive epoxy
A thermistor is a temperature sensitive resistor constructed of semiconductor materials with two leads
Units of measure
If water was most dense at its freezing point (0 °C) then it would sink to the bottom, freezing from the bottom up, killing all organisms.
Rather the bottom of a body of water remain at least 4°C, and thus unfrozen supporting life under the ice.
Freshwater is most dense at 4 °C
Learn more about water temperature at The Fundamentals of Environmental Measurements: Water Temperature