Submarine Attack puts you below the surface with a torpedo tube: enemy ships pass above, you aim, and you fire — accounting for the fact that a torpedo takes time to reach where a moving ship will be.
Leading the target is the skill. Aiming at where a ship currently is guarantees a miss, and the further away it is the further ahead you have to aim.
Because you're aiming at where the ship is rather than where it will be. Torpedoes take time to travel, so you have to lead the target — more so at longer range.
Yes — new weapons and abilities unlock as you win engagements.
Yes — hold and drag to aim, tap the red button to fire.