Included free with the base game — no Steam store pageIt launched as paid DLC in 2019, but has been integrated free into the base game for years now. It still earns its own file: the ship designer it introduced remains, to this day, the foundation of all naval customization.
What New Content It Added
The pack split its additions between two new focus trees and the first of the game's three big modular designers.
Nations: the US, the UK, Mexico, and the Netherlands
The United States and 
the United Kingdom debuted their first serious focus trees here, alongside smaller content for 
Mexico and the Netherlands.
Mechanics: the Ship Designer, Piece by Piece
The big new feature is the ship designer: instead of picking from a handful of fixed models, you build each ship class module by module — hull, armament, armor, systems. The DLC also brought governments in exile for occupied nations, naval mines, and naval treaties that cap your fleet's tonnage if you choose to sign them.
What's Left Today
It was the first of the game's three major modular designers — the tank and plane designers followed later — and it's still, without exaggeration, the foundation of how any fleet gets built in the current game. Being integrated free, there's nothing to buy, but it's worth knowing these mechanics were born here if you're running into them for the first time.