Hello,
A Mast Rigger is certainly not the same thing as an Admiral's Coxwain. The Admiral's Coxwain was first and foremost the seaman in charge of the Admiral's barge, taking him to the shore or other ships as neccessary and assissint the Admiral in any other way needed you could hardly do better than to read the "Bolitho" series of novels which give an idealised picture of life in the Edwaedian Navy.
Rick