I’ve spent a lot of time at the coast this summer and this got me curious. In England I’d say that the southwest has by far the cleanest. It helps that the region has hundreds of beaches so many don’t get visitors anyway.
I’d say the northern coasts are worst for litter. But it depends on where you are – if the beach is at a national park/famous with tourists, it’s likely to be clean – probably because more efforts are made to keep them that way. If it’s a beach that gets mostly local visitors (and especially if it’s in an area with lots of antisocial behaviour, which there are a lot) it tends to be bad.
I’ve not seen enough of the rest of the south coast to judge.
So how is it in your country?