If there's such a correlation, it's more likely to be indirect, with large urban areas being less religious, and the same large urban areas being likely to be sited next to a river or coast.

As a direct example, the province of Zeeland is often considered to be part of the Dutch bible belt, and it doesn't get much more coastal than that. It does lack a major city.

Interesting idea. Makes sense because coasts are where immigrants and foreign nationals first come to when they travel to the given country. This seems pretty spot-on. Or maybe it's because cities in general are developed alongside water bodies, and some of these are on oceanic coasts, and cities are more diverse in general, making them more liberal.

Some inland areas are more liberal than coastal cities, though (Paris is probably more liberal than a small Mediterranean city on its southern coast).