Many years ago (late 1940's) there was an article in one of the model magazines about small (6 or 8 inch wing span) model gliders that had a slight bouyancy and were released at the bottom of a swimming pool (upside down) to glide slowly to the top of the water. Small pellets of potasium permanganate were attached to each wing tip. The bright purplish dye left spiral tracks of the tip vortices that was easy to see. Potasium permanganate is quite poisonous and should be handled with skin protection.