Unfortunately that doesn't surprise me in the least. It seems whenever I'm near the sea, wherever in the world that may be, there are more plastic bottles than pebbles!
I remember being on the Isle of Skye once on a really remote beach a fair old way from any settlements (well let's face it there aren't that many people living on the Isle of Skye where there ARE settlements!) and there was crap of every description EVERYWHERE! It looked as if every person living on that island had gone out of their way to get down to that beach to dump their rubbish, which was highly unlikely given that the beach was a fair old slap away from any roads. Of course most of that rubbish had probably just been washed up there. It was just so sad. The smell was dreadful and their were rats scuttling all over the place.
It was the same when I used to live in Saltdean; rubbish everywhere on the beach - bottles, yoghurt pots, beer cans, bits of old fishing net, tangled lumps of rope, planks of wood - you name it. Sometimes I'd have a clear up operation and start bagging it all up but there'd be just as much shite knocking around the next day.
I think there's this awful 'out of sight, out of mind' mentality where people think if they chuck rubbish in the sea it'll just be 'eaten' by the ocean never to be seen again! All very depressing.