basically, yes, in most developed countries ships are provided with large amounts of fuel oil (bunker fuel) on credit, trusting that the large shipping company will pay in the very near future by wire transfer. In really sketchy 3rd world locations ships have to use cash from the captain's safe to buy fuel. If a ship is delinquent in its account with the local fuel supplier, the fuel supplier will file something in their local admiralty court for what is essentially a lien to seize the ship.