Hmm. Did some quick zooming in on Googlemaps, just to get a sense (looked at the eastern coast of U.S. and coast of France/Italy as examples). Looks like large numbers of coastal lakes aren't that uncommon. What I can't say is whether they are more or less common than inland lakes (that's harder to gauge, I think). On the other hand, what's also hard to gauge is whether what I see in Googlmaps are really just lagoons and bays and whatnot that are very highly sheltered.