I decided to go for a nautical theme across all my difficulty levels.
Fishing Marina:
Boat
Yacht
Trawler
Ship
Ocean
Ferry Terminal:
Captain
Terminal
Catamaran
Lifeboat
Steamship
Naval Base:
Agamemnon
Temeraire
Bellerophon
Dreadnought
Polyphemus
(These are some of Nelson’s ships at the Battle of Trafalgar)