How do Pip and Herbert plan to get Magwitch on a ship leaving England?

THey would sneak him down the river and eventually get him on a ship leaving England,

"We are both good watermen, Handel, and could take him down the river ourselves when the right time comes. No boat would then be hired for the purpose, and no boatmen; that would save at least a chance of suspicion, and any chance is worth saving."