Brewer's: Sea-girt Isle
England. So called because, as Shakespeare has it, it is “hedged in with the main, that water-wallëd bulwark” (King John, ii. 1).
This precious stone set in the silver sea, Which serves it in the office of a wall, Or as a moat defensive to a house, Against the envy of less happier lands.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894