Bretonia Here the idea is historic knights/paladins. Charlemagne's order is considered the original, Pendragon is from King Arthur's last name, and Durendal, the sword of Roland. (Though Durendal would be badass for a Rheinland ship name too.) I tried to pick names that make you think of Knights, and royalty.
Kusari These names are based on various Asian dynasties. Yuan from China, Joseon from Korea, and Azuchi from AzuchiMomoyama from Japan. Chakri is from Thailand, and is the name of the current royal ruling house.
Gunboat - Chakri
Cruiser that pretends to be a gunboat - Yuan
Battlecruiser - Joseon
Battleship - Azuchi
Rheinland For this, I continued the idea of Norse and Teutonic gods.
Gunboat - Baldur
Cruiser - Woden
Battleship - Freyja
And that's it for now I guess... If there's any comments, corrections, etc. I welcome them!
Gunboat - "Yorktown" class gunboat
Siege Cruiser - "Omaha" class siege cruiser
Battle Cruiser - "Alaska" class battle cruiser
Dread - "Delaware" class dreadnought
Carrier - "Nimitz" class supercarrier
Bretonia
Gunboat - "Dryad" class gunboat
Cruiser - "Vanguard" class cruiser
Battleship - "Colossus" class battleship
Kusari
Gunboat - "Chiyodagata" class gunboat
Cruiser that pretends to be a gunboat - "Chiyoda" class heavy gunboat
Battlecruiser "Takao" class battlecruiser
Battleship - "Mikasa" class battleship
Rheinland
Gunboat - Panzerschiffe (Armored ship)
Cruiser - "Kreuzer" class heavy cruiser
Battleship - "Deutschland" class battleship
//almost all of them are/have been classes of ships in human history
Bretonia: Military Victories (or at least not very botched defeats)
Gunboat: Dragoon
Destroyer: Vitoria (not a typo)
Battleship: Dunkirk/Victoria/Nessie (If you scan Bretonian battleships, these are their classes, personal preference is Dunkirk)
Rheinland:
Gunboat: Panzer
Cruiser: Valhalla
Battleship: Bismarck
Kusari: (I found a translator)
GB: F-0R "sencha"
Destroyer F-4R "kashou"
BS: 5-N4C "Jishaku"