Confirm that the temperature readings are realistic, without too much variation.
Temperature is measured by a thermistor (a temperature sensor) located inside the instrument head and reported in Degrees Celsius. The reason for measuring temperature is primarily to calculate the speed of sound, which further is used to convert the time between transmitted and received signals into distances. Measuring the correct temperature is therefore crucial for finding range and cell locations.
The water temperature varies with salinity, latitude, and depth. For this reason, the given position of an instrument during deployment will determine which temperatures are realistic. There may also be periodic differences throughout the day, because of seasonal changes or due to external factors. Unrealistic temperature measurements may indicate that something is wrong, either with the sensor itself or the measurement conditions. A sudden change in readings of temperature can be caused by something blocking the sensor, such as organisms, biological material, rubbish floating in the water, etc. If this blockage eventually disappears, the temperature will return to the excepted values because the sensor will then measures the water temperature again (and not the thing blocking the sensor).
Updated