Buffer is the colloquial title for the senior seaman sailor in a Commonwealth of Nations navy ship.
In smaller ships, such as a patrol boat, the buffer may be a petty officer or leading rate.
The buffer reports to the first lieutenant or deck officer, and has a wide-ranging roving commission to supervise seamanship evolutions (activities) and issue directions to seamen as required, and advise "part of ship" officers and petty officers on their activities.
The buffer will supervise major ship activities, such as: berthing alongside and taking on equipment in harbour; anchoring, mooring and weighing; rigging for refuelling or stores transfer at sea; sending away and recovering a sea-boat.
The equivalent position in the United States Navy would be that of a command master chief petty officer in the boatswain's mate rating.