From Red Right Returning:
The USCG has been contemplating combining all your credentials into one place, and an idea that is gaining traction is a "passport" style booklet.
Non-U.S. merchant mariners often have a seaman's book instead of the U.S. style card. The advantage of a book is that it is stamped with the ship's stamp, each mariner is associated with a ship. Your ID shows what ship your are on. U.S. mariners often have to carry two IDs, both the seaman's card (Z-Card) and a ship's ID.
Stealing a line from RRR's Capt Douglas Gould - You saw it here first (unless you saw it someplace else already...)