I think I saw a boat pick it up halfway to the freeway, try contacting Matilda bay yacht club, definately saw the kite taking off away from the sandbar, hope you find your kite.
make that royal perth yacht club or mounts bay sailing club (no such thing as matilda bay yacht club) sorry to nit pick but he wouldnt have been able to find it in the white pages hehe and goodluck kd
Might be an idea to ring Marine and Harbors as they are the police on the water and if it got picked up that might be a logical place to hand it into . good luck