I don't think it's accurate to say that all coastal towns in the UK are doing badly - I can't help noticing there here on the east of Scotland you have on different sides of the Firth of Forth Methil, which is deprived, and North Berwick which is doing very nicely.

Different histories, reliance on different industries, quality of transport links make a huge difference.

Not even all coastal "tourist" towns are doing badly - I live near Burntisland in Fife which actually seems to be thriving, probably because of the excellent location for commuting by train into Edinburgh, combined with nice beaches, great views (of Edinburgh!)...