In the law-courts of the UK you are placed in a “Dock”...?

...a friend of mine suggested this is because we are tried under the "laws of the sea" (according to Magna Carta) and that should you ask the judge if he is under oath...as he will not be, he must therefore try you under the "laws of the land". As these laws are common-law and therefore anarchy - when were the "laws of the sea" repelled?