Sea coal was occasionally found on the shore near my old home town on the Forth. Apparently the very start of the local coal industry was back in the middle ages when local monks would forage for sea coal along the riverside. I believe that it may have been the result of a seam of coal along the riverbed being exposed, and the coal breaking up and washing ashore. Local folk believed that "Sea Coal" was a far superior fuel to the usual stuff - probably because it was free!