I think this term describes ordinary sailors who declaimed to their crewmates about their rights, etc. under British Navy rules.
"There were several recent draughts; and there were some King's hard bargains, including two or three sea-lawyers." --Patrick O'Brian, The Nutmeg of Consolation, 10
Here's another passage that seems to explain it, but doesn't use the term specifically: "'...whenever a King's ship is lost there are always a few clever fellows who tell the rest that since the officers are commissioned to a particular ship they have no authority once that particular ship is gone. They also say the seaman's pay stops on the day of the wreck, so no service or obedience is due—the Articles of War no longer apply.'
"'Are these things true?'
"'Lord, no. They were once upon a time, but that was knocked on the head after the loss of the Wager in Anson's day...'" --p. 23