any of a class of soldiers serving on shipboard or in close association with a naval force
seagoing ships of a specified nationality or class
the mercantile marine
tell that to the (horse) marines an indignant expression of disbelief at something said
[said to be associated with Samuel Pepys, who when he mentioned his belief in flying fish received support from an officer of the Marine Corps; the King (Charles II) believed him since only the marines had the necessary knowledge of far places, and anyone who doubted a similar story in future should first ‘tell it to the marines’. The variant form horse marines adds an element of fantasy with the notion of horse soldiers serving on a ship.]