These are to indicate when a ship is fully laden, and this depends on its destination. F.W. (Fresh Water line), i.e. it may be laden till this mark touches the water when loading in a fresh-water dock or river.
I.S. (Indian Summer line). It was to be loaded to this point in the Indian seas in summer time. S. The summer draught in the Mediterranean.
W. The winter draught in the Mediterranean.
W.N.A. (Winter North Atlantic line).
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Ship Letters from Infoplease: