Wait, what does "humanity also doesn't exist inside of their memories" mean? Is it supposed to be taken literally?
This fiction already is based upon the notion that they're time travellers/sliders/whatever that go between alternate timelines of parallel dimensions, so sure, why not? This is an alternate Earth with shipgirls but no humans. (And considering the peacefullness, possibly not even any abyssals.)
Yes, the history of this world is the opposite of ours.On the other hand, in their world, there is no Allied or Axis nations, with the slight exception that humanity also doesn't exist inside of their memories.An ideal world, yet it is missing something.A beautiful, yet empty world.Hyuuga.Hey, Hyuuga!It's instructor Hyuuga.Instructor Hyuuga! You should teach us how to fight more! Like how to kick, or punch!... Yours is pulling.Ohh!?A quiet, yet somehow too quiet sea.It is a world where the Great Japanese Empire's Combined Fleet and the American Seventh Fleet, as well as the Queen Elizabeth and Bismark class ships are working together.Even catching a fish... is a good way to train your patience and concentration.