The weaker versions tend towards being all black armor plates with fish-like forms, while the more powerful, "evolved" forms tend towards being humans with large black metal guns attached. In-between ships tend to have a human part (like just the torso or just the head) leading into metal-skull-crab parts.
Just as player shipgirls take inspiration from real WWII Axis warships, many abyssal boss or event-type characters seem inspired by real-life bases or ships from the Axis or Allied sides. Earlier bosses notably were more Allied (including Port Darwin as Seaport Princess or Isolated Island Oni as Wake Island) while more recent bosses are clearly meant to reflect dark versions of Japanese ships (Aircraft Carrier Water Oni as "dark Shoukaku" or Destroyer Princess as "dark Harusame").
Abyssal ships were designed and illustrated by Oguchi (regular enemies) and Akira (Kadokawa) (boss characters & submarines). Most of their model numbers are based on the Iroha order.
