A small harbor for boats is called a marina, and consists of individual berths where boats can dock. A cabin is a room on a boat or ship, and usually contains berths for passengers to sleep on. On a cruise ship, an outside cabin has a window and small round windows on ships are called portholes. Ferries where vehicles drive on at one end, and then drive off at the other end when the ferry docks are called roll-on-roll-off ferries. Embark is a verb which has the same meaning as to board or get on a ship or aircraft. The opposite is disembark. Seasickness caused by the movement of a ship can be relieved by taking pills. There is an excellent ferry service between the islands, with eight crossings a day in the summer. Many ships sail under a flag of convenience, which means that they are registered in another country from that which they operate.