I thought the pic looked really odd until I read your post. makes sense, a lot of times old construction rubble is used as riprap to prevent erosion/build jetties. So, a lot of building stuff ends up in the ocean, and some will get broken/weathered loose, tumble around a bit, and show up somewhere else. (or it could've just washed down a river, been dumped at sea, whatever...)
Since you seem to have an idea of what it was intended for, any idea of what it's composition is? I'm not having much luck guessing at it's composition from the pics. do you think it's cut/shaped stone, or something akin to brick, terra-cotta, cinderblock, concrete/cement?
Maybe if you could get a macro/close up of the piece itself, so we could take a guess at it's composition/qualities?
It looks a lot like a molded cement, however you see that black stripe down the stone long ways? i have no clue what could have made it like that, i think it may be clay because of how that stripe goes down the middle