This study reports a waterflow sensor for marine biologging and examples of its application to sea turtles. The relative waterflow velocity is one of the most important indicators for evaluating the swimming movements of marine animals. Propeller-type sensors have been used as conventional sensors for marine biologging. However, the sensors' moving parts are easily affected by suspended marine debris and attached organisms. Therefore, long-term and high-temporal resolution measurements of the velocity have been challenging. Therefore, we have developed a pitot-type waterflow sensor that enables long-term and high-temporal resolution measurement. A rubber film is glued on the inlets, and silicone oil fills inside the flow channel. The film prevents debris from entering, and the oil prevents sensor destruction due to water pressure. In addition, the sensor detects waterflow by measuring the flow-generated pressure without moving parts. These features provide the sensor with high resistance to the harsh marine environment and enable high-temporal resolution measurement. Finally, we experimentally proved that the developed sensor can be used for biologging. This sensor was connected to a logger and attached to a wild loggerhead turtle. The turtle was equipped with parachutes that generated drag force. The relative waterflow velocity of the turtle was measured for two hours, and the parachute was detached one hour after the start. The sensor measured the waterflow with a high-temporal resolution of 6.25 Hz. Furthermore, we were able to measure the change in the turtle's velocity due to the attachment and detachment of the parachute.