In my experience, the most common loss/victory is through surrender, typically following a bad defeat. For example, if the US or Japanese fleet were caught in a death trap and annihilated due to careless play, the corresponding player would probably give up. I’ve seen this many times.
Only once I have actually achieved victory conditions (and I did it in six rounds). Even then, my opponent surrendered rather than play the rest of the 6th turn because he had no realistic hope of freeing a victory city.