One of the few things that are worth a visit in Cherbourg, or even doing a detour to visit, is the Cite de le Mer museum. The museum is quite recent, it opened its doors in the year 2002. But the building is not new. It is the former transatlantic terminal building from the glory days of steamships crossing the Atlantic.
The terminal building was heavily used between 1930 and 1960 although the building itself dates back to 1858. Many of the luxury steamships and later cruise ships arrived to the mainland Europe at Cherbourg, including the ill-fated Titanic.
When you arrive at the museum, just in front of the building, you will see a “souvenir” of the WWII;

…a bunker made by the Germans. The bunker can not be visited.

Close to the main entrance of the museum, alongside the seawall, you will find this monument dedicated to the fishermen who lost their lives at sea.
Entering the main doors of the museum, but before you have to pay your entrance you are already surprised with the displays there (these are free, you don’t have to pay yet).

There are several exploration submarines on display here. These are the submarines used by scientists to explore the depths of the oceans. The submarine in the photo above on the right is the Archimede, capable of diving up to 11,000 meters (36,000 feet) depth.

Several of these submarines are on display. Then after paying your admission, you are confronted with this submarine:

It is the decommissioned Redoutable, a French navy nuclear submarine built in 1971. It was actually built here in Cherbourg.

The submarine was decommissioned in 1991 and starting in the year 2000, it took 2 years to transform this enormous submarine into the museum exhibit (and central piece).
This submarine is the biggest submarine accessible to the public in the world. It is also the only nearly-complete (the nuclear part has been removed) ballistic missile submarine hull open to the public.

Entrance to the sub is via the engine room. Initially it doesn’t look that big, but when you walk through the very narrow and low hallways, you get to see that the engines are enormous.

The engine takes up a very large portion of the submarine.

The hallways are all very narrow and low. My 1M94 height had problems moving around and I needed to pay attention to not banging my head. With my rucksack it was impossible to turn.
NOTE: I believe there is a separate visiting-circuit for wheelchairs, since I saw a door marked with the wheelchair symbol.

These are the officer sleeping quarters hallway, nicely done up in wood; the crew aren’t that lucky…

The combat station and operational parts are all dowsed in red light.

This is where they “steer” the sub, two seats with yokes controlling the direction of the vessel.

The crew on this submarine are obviously selected on their height. What you see here is not the broom closet but the Captain’s quarters and bed. The bed is about 1M70 – 1M80 long (about 6 feet long). Not at all as you can often see in the movies where there is room for several officers to discuss things with the Captain. Here there is only room for one person..
Some of the officer’s bedroom were the same size except they have a bunk bed (2 to the same room – yikes!).

The officer leisure room, where they spend their non-active hours looked cozy. But the crew did not have such a place. Instead they can stay in…

… the eating area. In total the Redoutable carried 15 officers and 120 sailors.

The sailors slept on triple bunk beds located alongside the hallways. No privacy for them (there are curtains).

At the front of the 128 meters (420 feet) long ship you will find the torpedo room.
There’s also a large display section inside the building explaining submarines and their history.
But that’s not all there’s to see. Apart from water-based virtual reality activities building and aquarium, there is also a large portion dedicated to the Titanic.

This for example was the room to be used for the Titanic for passenger arriving with their baggage (a kind-of checkin facility). If I understand correctly, it was recently used for the Queen Mary II.

Above is a replica of the 3rd class cabin onboard of the Titanic. It was considered one of the most luxurious 3rd class cabins of those days.

And this is the First class cabin. Not that shabby.
You can easily spend a whole day in this museum, there is a lot to see.
Conclusion
This is a museum both young & old will enjoy. How often do you have the chance of being inside a nuclear submarine?
TIP: Arrive early if you want to see the sub, since the queue quickly grows.