Water flow is strongly related to swimming speed; thus, technologies for measuring the three-dimensional movement of water are highly desired in the water sports industry. However, existing fluid measurement methods are not suitable for use with humans because they introduce tiny plastic particles (known as tracer particles) that contain fluorescent ink, which is used to visualize water flow. A laser then irradiates the environment to make the particles brighter than the surroundings to track their movement with cameras. This method has potential adverse effects to humans, such as accidental swallowing of the particles and laser burns to the skin and eyes. In this research, we propose a human-friendly water flow measuring technology using tracer particles made of food-grade materials and a harmless light source. To visualize tracer particles, we give the particles an optical property, which makes them sufficiently brighter than the surroundings when placed between circularly polarized plates. We tested the proposed setup for water flow measurement in an actual swimming environment with swimmers. We observed that tracer particles moved in accordance with the water flow caused by a swimming stroke.