Something I'd wondered about and found out the answer on the weekend.
Short answer: you swim, and swim, and swim, and swim trying to catch up with it and then if you're very lucky, as I was, someone helps you out and drags it in for you.
I was riding toe side about 4-500 metres off Mullaloo, went to turn back to shore when my harness (Dakine-seat) came unhooked at the crossbar...WTF.
I released the safety and let the kite look after itself while I loosened the straps on the harness so I could get it hooked up again and while I'm doing this somehow have pulled on the velcro release for the safety leash...NNNNOOOOOOO!!!! Kite disappears very quickly downwind until it settles leading edge down and starts to drift just a tad slower than the speed I could swim at.
I reckon I might have caught up with it eventually but don't know if I would have been in any state to do anything with it. As it was my calves were starting to cramp up pretty bad by the time I aborted and got back to the beach without having rescued the kite.
Huge thankyou to Rob Gannon who teaches at Mullaloo and incidentally gave me my beginners lessons a couple of years ago. The prospects of getting back to the beach with kite and board (and me) all present and intact were looking pretty slim until he came along. The appropriate reparations will be forthcoming. Thanks again.
J-P