Those underwater craft are used to launch ballistic missiles 1000s of miles away. And underwater combat is pretty much the opposite of space combat.
Sorry bro, I know I'm coming in late to this part of the thread but underwater combat is exactly what space combat would be like, barring magic FTL/inertia dampeners etc bullshit. Self-contained vessels that need to be able to operate independently for months in a terrifyingly vast 3D environment where stealth is only possible through careful exploitation of rare(ish) geological formations (planets, moons, asteroids, Sun), hoping to spot the enemy before the enemy spots them in-order to launch missiles millions of kilometres away before running into another hideout. Missiles that only need to engage their engines for few minutes to accelerate to a sufficiently high velocity, then can coast along trajectories and save the remaining delta-v for course corrections when they get closer to their target - which cannot evade aggressively enough without killing the crew aboard.
Position attacking craft directly between the target and the Sun and there's no way the target would even spot the launch of the missiles as any thermal/optical sensor would be blinded by the Sun. Building long-distance loitering missiles is already perfectly possible - check the British ALARM as the perfect example.