Storms are not exactly a rare thing in the English Channel. Let us say that in September, 1066, a sudden, rather nasty storm pick up in the Channel, during the crossing, and William is swept overboard and drowns, along with a number of other people in his fleet. In the confusion, some ships head for the British coast, others return to Normandy, and a few sink. This has no effect on the Battle of Stamford Bridge (which, IIRC, took place a few days earlier), of course - Harald Hardrada still dies on schedule.
So, what other challengers, if any, are there for King Harold Godwinson? What sort of king will he be, with this apparent miracle saving him from the Norman invaders?