Naval Modern sailors' slang for the sea; it is said to be derived from Hogwash, though some assert that it comes from a mispronouncement of Ocean. Synonyms are The Ditch, The Pond, The Drink, all three of which words are used by officers more often than Oggin. 
used for water in general, it came about from the water containers during the war having the word oggin written on them.
Oggin is British navy slang for the sea.