thumb|Toilet on M/S Diana, a Swedish passenger boat built in 1931
In sailing vessels, the head is the ship's toilet. The name derives from sailing ships in which the toilet area for the regular sailors was placed at the head or bow of the vessel.
Design
In modern boats, the heads look similar to seated flush toilets but use a system of valves and pumps that brings seawater or fresh water into the toilet and discharges waste into a holding tank or out through the hull. In small boats the pump is often hand operated. The cleaning mechanism is easily blocked if too much toilet paper or other fibrous material is put down the pan.
Discharge of sewage from boats is regulated by international agreements and national laws to reduce marine pollution, including the International Convention for the Prevention of Pollution from Ships and the United States Clean Water Act (see regulation of ship pollution in the United States). Marine sanitation devices treat, process, and store raw sewage. Modern boats commonly have a holding tank so that sewage can be stored and later discharged at a sanitary pumpout facility on shore or discharged into the sea at a legal distance from shore. The captain and officers had more private toilets at the stern of the ship in the quarter galleries, although the mechanism was the same. Officers also used chamber pots.
18th century naval ships had similar facilities, although by the 1790s some ships may have had a few flush toilets. This caused the loss of . The toilet on the World War I British E-class submarine was considered so poor by the captain of that he preferred the crew to wait to relieve themselves until the submarine surfaced at night.
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File:Heads on HMS alliance.jpg|Head in British submarine (c.1945–1973)
File:FoxTrot 480 0071.JPG|Head in Soviet submarine of the (c.1957–1983)
File:USS Growler - Toilet (7181617746).jpg|Head in submarine (mid-1950s)
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