Referring to Others

The idea of this is to (hopefully) lessen confusion a little.

Other Ranks - Riflemen, Chosen Men, Privates (either soldiers or Marines), sailors
Referred to as Rifleman/Private/Seaman
Addressed by rank or surname only. Close friends would be more likely to use first names between themselves. Officers can use first names, but are unlikely to do so

NCOs - Sergeants, Corporals
Referred to as Sergeant/Corporal
Adressed as Sergeant Smith or Sergeant/Corporal Smith or Corporal. 'Sarn't' and 'Corp' are shorter forms.

Officers
Referred to as Lieutenant/Major/Colonel Smith, or Mister Smith (if below Captain in the Navy and Major in the Army)
Addressed as 'sir', by those lower in rank. Officers of equal or higher rank may use the rank (i.e. Captain or Lieutenant) as a friendlier form of address that is more formal than using the other's name. First names only used between very good friends.

Civilian
Civilians who are gentlemen would be referred to as 'Mister' and addressed as 'sir'.
Civilians who are not gentlemen would be referred to and addressed by surname only, or possibly first names in the case of servants.

I don't know about petty or warrant officers in the Navy, but they are all NPCs at the moment anyway.

What's the difference between a petty and a warrant officer? I never can remember.

I think the carpenter in the movie of Master and Commander gets referred to as "Mister Lamb." And I think Billy Budd's master-at-arms gets referred to as "Mister Claggart."

PS - What does NCO mean? Non-commissioned officer? What does that mean?

An NCO is, as you have it, a non-commissioned officer.

A commissioned officer is one who holds a commission from the King, and they are only given to gentlemen, as a regular thing.

An NCO is one who is in charge of a squad of men. They usually are the ones to be found taking drill and arms training.

A warrant officer holds a warrant from a Board. They include the master, the surgeon, the cook, the bosun - important people. Petty officers are promoted - 'rated' - by the Captain of a vessel, and only hold that rank during that ship's commission, and they can lose the rating, and thus the rank, at the Captain's discretion. A warrant officer can't.

I think the more important Warrant Officers, such as the Master, are generally called 'Mr' - This is quite useful (and seems to accord with the little I know) about warrant officers.