Instead of a naval unit that has to break the game mechanics for viable play, what about a new surface combatant like the British MTB, American PT Boat or the German E-Boat?
You could have a squadron of 5 torpedo boats in a unit, each able to launch one torpedo per attack. With a communal hp pool. Every time you lose 20% of your hp, you lose a unit and reduce your torpedo salvo by one.
The boats could have close to medium range AA, high speed and the ability to generate engine smoke for defence. The other ships could have a consumable that allows you to retask the secondaries and AA for defence against the torpedo boats.
Obviously such light craft would be incapable of inflicting ramming damage on larger ships. And if they become the new primary cap circle ships, they should be able to retask their light to medium AA weapons against each other.
The torpedo boats would keep everyone on their toes and you could use the smokeless DD gunboats as torpedo boat hunter killers. Given effective weapons for the purpose.
I've only been playing for a few weeks and wondered about all of the anti-submarine consumables on the regular ships. Yet not one of them has the hedgehog mortar. And it's apparently really hard to kill a sub in play.
We can't forget that hunting subs in the later years of WWII gave advantage to the hunters, even with the innovations like the snorkel. Airborne radar and ASDIC, along with more effective weapons like the hedgehog, turned nearly invisible (and invincible) submarines into death traps for their crews.
If I wanted to play submarines, I would get a submarine game. I would not take a reasonably functional surface warfare game and break its mechanics to introduce them. Just my two cents worth.