This study presents a Pitot-static-tube-based waterflow sensor for biologging of marine animals. To attach the sensor on marine animals, the relative waterflow measurement becomes possible during swimming. The sensor is composed of two housing parts and sensor element. Incompressible liquid is sealed inside the sensor housings to realize high-pressure resistance. A thin film is glued inlets of the sensor to prevent air bubbles invention. A membrane-type differential pressure sensor element is built inside the sensor to measure differential pressure between each inlet. The developed sensor responded to waterflow velocity from 0.2 to1.6 m/s with sufficiently high sensitivity.