Know Before You Go
The size of the U-boat pen is not to be underestimated. It is HUGE. You can drive past this concrete monolith (at high speed) for about 30 minutes. It is too far to walk from the town.
The Port Authority considers the structure derelict and unsafe, hence they are not open to the public. They are, however, visible from the adjacent road and hard to miss.
The U-boat complexes in St. Nazaire, Lorient and Bordeaux are open to the public. The pens at Brest are still used by the French navy and remains closed to the public except on certain days when it (the base, not necessarily the pens) is open to citizens of E.U. member countries only.