There is limited experimental data describing the mixing processes and coherent velocity structures near the surface of the ocean. These play an important part in the interactions between the atmosphere and the ocean and thus affect the climate of the earth. A robust, low cost thermistor based sensor suitable for use in the detection and quantification of velocity structures has been designed and developed. The flow sensor, a thermal anemometer, consists of a self-heated thermistor that is maintained at constant temperature using a feedback control circuit. The thermistor is exposed to the moving fluid and the heat transfer from it is a function of the velocity of the fluid. The sensor has been interfaced with a PC to facilitate data acquisition. A tow tank calibration and testing facility also interfaced to a PC was developed. The sensor is compensated for changes in ambient fluid temperature, which is a major problem for all thermal anemometers that operate in water. The calibration relation that is normally used for thermistor anemometers has been improved upon and gives better results than any found in the literature. In order to provide electrical isolation from the water the sensor was protected with an insulating coating. The effect of coating thickness and the type of coating used on sensor performance was investigated. It was found that a polymer coating resulted in a sensor that did not show any appreciable drift due to sensor contamination over a number of months. This is a significant improvement over glass coated hot film sensors, which are widely used for velocity measurements in water and previously developed thermistor based sensors. The effect of various control circuit parameters on the frequency response of the sensor was determined and these parameters were tuned to give maximum frequency response A procedure for easily replacing broken/damaged sensors in the field was also developed.