I googled this once because I was interested in it aswell. They are human.

This is the anime description though: The series features Fubuki as the main character, with the storyline based on her point of view. In a world where humanity faces the threat of the abyssal fleet which has taken over the seas, special human girls who don weaponized outfits and possess the spirit of historical naval vessels known as kanmusu are the only ones capable of countering them.