Barracuda Submarine

The Barracuda runs submerged and is invisible to everything except sonar. Its torpedoes hit surface ships very hard, and nothing without sonar can even return fire.

How to use

Slip a few submarines into the enemy's shipping lane and sink isolated landing craft and surface ships as they pass. A fleet without sonar cannot detect the ambush. Once you see an Aegis, pull back or focus it down first: it carries sonar and anti-submarine torpedoes. The Barracuda cannot attack land or air targets; the submarine that strikes the land is the Kraken Cruise-Missile Submarine.

On the surface

T brings it up to run on the surface, and T again takes it down. Up, it is 30% faster, sees half as far again, and is a ship like any other: seen without sonar, shot by anything that fires on ships, jostled by the hulls round it. Surface to cross your own waters quickly, and dive before anything of the enemy's comes near. In shallow water (off a shore, in a river, at your own yard) it cannot dive at all: it runs on the surface there whatever you told it, at the slower pace every hull makes in the shallows, and goes down on its own once the water deepens. Hunt in the deep.

Anecdotes

Real submarines almost never use active sonar: a ping tells the target exactly where you are. Films are full of the sound; actual boats mostly just listen. The Barracuda's crew agree, which is why the Aegis has to come looking.