Nice work! I don't know how he
did it, but my approach would be a wireless waterproof camera
oriented to look back down the line. Zooming would just be the distance between kite and boat, and centering for the compilation would be the result of editing out the bad bits.
Wireless video is cheap
, and sealed cameras are small and light - see Supercircuits
and other sources. I'd probably be inclined to use a separate sealed camera and a low-power transmitter in a homebrew enclosure with battery
for both, keeping weight to a minimum. These folks
have lots of suitable kites; I use one for kayak propulsion