The new girl's name is Kanade, not Haruna as I erroneously noted down. Haruna is her sick older sister then. She is a noble on a quest to find a cure for her sister, I'm guessing.
Hinomoto. As a phrase the similar sounding, "Hi no Moto" (日の本) means Origin of the Sun, which is blatantly and on the nose name for isekai Land of the Rising Sun.
That attack has to be orchestrated. If regular attacks at sea were as powerful as that, ships like this wouldn't be plying this trade route anymore. Kanade being an archer is very convenient for this attack, since she can fight the sea serpents at a distance and not wait for it to get close or have to jump to attack. I guess she gets a turn to shine next chapter?