Submarine hunters are T2 units designed to destroy submerged units (submarines). As such they are all armed with torpedos and anti-torpedo weaponry to effectively attack enemy submarines but have no other armaments. All submarine hunters, excluding the Cooper, can submerge to avoid anti-surface fire.
In practice, other than the UEF T2 Torpedo Boat, Submarine Hunters are essentially Tech 2 Attack Submarines, and can be used accordingly.
The UEF T2 Torpedo Boat: Cooper is the cheapest of the submarine hunters, but is also the only one which cannot submerge, a significant disadvantage. It is, however, reasonably tough and powerful for its cost, and its torpedoes have a slightly greater range. Its anti-torpedo system fires faster than the Barracuda's or Vesper's. Its really more meant to compensate for the lackluster torpedoes of the Valiant-class than to do the same jobs as the other two.
The Seraphim don't technically have a T2 Submarine Hunter, but they do have two units which serve broadly in this capacity. They have the Uashavoh, their T2 submersible Destroyer, and they have the Yathsou, their T3 Submarine Hunter. Arguably, between the two of them, the Seraphim have the best "submarine" attack force, despite their having no one unit at T2 dedicated to this.