Okay, pretty sure that castle is in the Mediterranean somewhere, just basing it on the style of the crennalations. Where is that?
Well, that explains why I thought it looked Arabic. I was off on the location by a continent though...
Off the coast of France, perhaps? That is quite the level of aging and damage. Makes me think it has been there a while...
There is decent chance it was placed to seed reef and shallow water life. Military surplus is sometimes deposited at sea to draw reefs and fish.