Quote: na·vy (nv)
n. pl. na·vies
1. All of a nation's warships.
navy [ˈneɪvɪ]
n pl -vies
1. (Military) the warships and auxiliary vessels of a nation or ruler
Just to make sure everyone understand what the word "navy" means.
First of all commonly =/= always so your first argument doesn't stick.
The Corsair Empire is a nation and has warships, so all of the Empire's warships = the Empire's navy. The second definition includes the word "ruler" so you don't need a highly organized state-like structure to call the total of your ships a navy.
Therefore, C.N.S. makes A LOT OF sense.
(We) Corsairs do have a navy. So does every other group/faction/organization/ruler/warlord/nation that uses (space) boats.