According to the 'Natural Arch and Bridge Society' they do turn up all over the world, even in deserts.
Well, we must not jump to the conclusion that the desert ones are natural. If we found, say, that they all occurred in the Western United States, I would take that as highly suspicious. From the name of the society, I would not expect much help from this quarter.
I beg to differ. Yes for sure there are relationships here, but what came first? The megalithic Western Coast of Europe is also defined by extreme tidal ranges (in comparison to most other coasts).
This turns out not to be the case. Yes, some Megalithic sites eg Channel Islands, Mont St Michel, Bristol Channel, have large tidal ranges but other Megalithic sites (quite nearby) curiously do not. For instance, the most famous sea-arch of all, Durdle Dor, is down the coast from Poole Harbour which, I am assured, has a very small tidal range (not to mention four tides a day). Since presumably arches are a good indicator of the state of the tide -- and why is Durdle Dor always visibly an arch if there are thirty foot tides in its vicinity? -- then some correlative work is called for.
Actually, come to think of it, why do arches so often appear at exactly the right height? Shouldn't a whole bunch of them be too low (underwater) or too high (on land).