noun Coal from the sea: mineral coal that washes up from the sea onto beaches.
noun Coal from across the sea: mineral coal, as opposed to charcoal, in a time and place in which the former arrived by ship and the latter arrived overland (such as London in Elizabethan times).
noun Coal to be used at sea: a certain class of mineral coal, especially suitable for the steam engines of ships at sea and locomotives.
noun Such coal used in foundry practice, intermixed with foundry sand or applied in a layer on its face, to modify the behavior of the molten metal.