Definition of sea card:
(n) :
An ID card that identifies someone as eligible to work as crew aboard ship.
(n) :
An ID card issued by a cruise ship line to passengers, often tied to a credit card, which passengers use to board the ship after excursions, to enter their rooms, and usually to pay for items while on the ship.
(n) :
A map of the ocean.
(n) :
A mariner's card, or compass.