Puzzled.

Beach dog walk this morning. Uneventful unless you count the moment of excitement with a French bulldog. Anyway, the beach had washed up these yellow key things, perhaps a couple of dozen we spotted, probaly many more. A little over 4cm in length, an inclined helix shaft, a plated head that is a likely a finger grip and a serated top edge. The thread is rounded which suggests it is screwed into something quite soft and will provide low draw strength.

Any ideas as to what it is for? I am at a loss.