GoPro3 has an App that one can use iPhone to preview from the camera with WiFi, when I put camera in the water, the wifi signal is lost. Since there is ocean current, the fins have been moved from where I lost it.
I was expecting you to look at the film on a computer after completing the grid. You should be able to detect where the foil was from the time on the film.
If you had a waterproof gps you could film it at the end of each straight run of the grid. Otherwise you could film two lineups on the shore at the end of each line.
You could have 2 balloons to anchor one at the start of each layline, then aim maybe 40' down-current of the other balloon. Then at the end of each run, move the nearest balloon 80' down-current.
If you know which direction the foil would be taken by the current, you could start before the likely drop point and gradually lengthen the distance between the bouys every time you move the layline in the direction of the current.