Before the squall line moved in, there was one other kiter out with him and they were attempting to get back on the beach and land their kites. .
This is the real issue. If you are out on the water and see a squall line coming don't try and race it back to the beach.
The only safe way in this situation is to bring the kite down so that a wing tip is in the water ( to keep your speed slow ) and body drag in keeping the kite wing tip on the water, and when the kite reaches the beach while you are still in the water release the kite to safety.
Use an Oh shit handle held with one hand when you release your QR, then if it all gets ugly you can let go of the Oh shit with out having to find the leash QR.
Why this is the only safe way in is that as you get closer to land and buildings the wind increases and becomes more gusty.
The above safety will work for all styles of kites , and I have used this myself several times over the years and know it works, so be safe if you get caught out in squally conditions.