That would make sense and why his dad is convinced he attempted a swim. So he had the rucksack which was attached to him as he swam. It eventually washed ashore and was later discovered by the deer stalkers.I assume he had the backpack with him and it was washed up on the shore, not that he left it on the shore and continued without it.
Possibly with a float attached or inside the backpack as many wild swimmers use.