We propose a potential fluid-measurement technology aimed at supporting biomechanics research of water sports using fluid simulation and motion analysis. Cellulose nanofibers introduced into the water as tracer particles to visualize the movement of water. An optical property of nanofibers, called flow birefringence, makes water flows brighter than their surroundings when placed between right and left circularly polarized plates. We tested the capability of the technology in a water tank and succeeded in using an existing particle-tracking method-particle image velocimetry (PIV)-to measure the flows from a pump in the tank.