The temperature of the water affects many other variables, partly directly through the motion of the atoms, such as the ability to conduct an electrical current and the water's ability to dissolve other substances, such as carbon dioxide and oxygen, and partly indirectly as the biological activity depends on the temperature. The values from measuring pH and conductivity are corrected according to the temperature, and many chemical analyses must be performed at room temperature (20 °C). The thermocline in lakes and oceans is of great importance to the turnover of the water masses (see "sea in constant motion"). Measurement of the temperature in seas can be properly done with a thermistor (or any other heat sensitive electronic component) with a long wire, connected to an indicator instrument. This thermistor is lowered (fitted with sinkers) into the water and the value is typically read every one metre. However, shorter intervals above and below the thermocline and more frequent intervals within the thermocline is appropriate.

Due to safety issues, mercury thermometers should be replaced with alcohol thermometers or with digital thermometers.