An Admiral's Hat (Also known as a Bicorne), is a historical navy officer's hat, usually worn by Admirals and other high-ranking commanders.
It is a sign of office and is often black or blue, with gold plumage on the crown.
All Admirals within the Grand Alliance Navy would own one such Bicorne and would wear them on formal events, certain military marches etc.
Some Admirals even use it in battle!
The Honour Guard
The Bicorne is an almost compulsory item within most formal dress uniforms, along with medals and/or special sashes.
The Stormwind Human Admiral uniform generally consists of a blue and gold Bicorne, shoulder belt and/or Aiguillette.
The Gilnean Uniform is a little more ornate, consisting of blue & gold (Sometimes grey) Bicorne, shoulder belt and an Aiguillette. They also wield a rapier, usually gifted at the same time as the Bicorne, as both are Admiralty badges of 'office'.
Kul Tiran Humans
The Kul Tirans are famous for their seaweed green Bicornes and uniforms.
It is unclear whether or not Draenei are active within the Grand Alliance Navy and if they have their own unique Admiralty uniform. (Would most likely follow Human Dress code to some extent.)
Dwarves and Gnomes
Dwarf and Gnome Admirals would most likely (If at all) follow similar Alliance dress codes as both populations aren't as sea-faring as other Alliance Factions and would most likely not have one of their own navies because of such fact, but would serve on other Alliance vessels (Generally.)