According to Hashtag's Guide, Gun whales are enormous gap-mouthed beasts that look like very big fish with half of a ship’s mast strapped to their spines. They can be blue, black, gray or white. They are not quite fish, as they breath air and prefer to stay close to the surface. A ship's crew can hunt and harvest one, resulting in enough food to feed the entire crew for several turns, but the damage a ship took from fighting one would not be worth what was saved in upkeep.
The “mast” part of the beast is a long twist of bone, as thick as a man’s chest, which fires Shockmancy bolts just like any naval beam weapon. Although anchored to the spine, the “gun” juts out a little in front of the creature’s head, allowing the gun whale to stab its opponent and shock it directly, even under water.