They would be cool. And the water doesnt have to be "deep" to use them. Simply make it when surfaced, have a shape like a surfaced sub. Give it a deck gun and a couple AA guns. And a wake when moving like the PT.
When submerged, it would be invisible. Unless the captain raised the scope. At that time a scope with a wake(if moving ) would appear. The sub could have 4-6 tubes fore and aft, with a reload abilty after so many minutes.
The scope view would be like a gunsight on a tank. No complicated range/bearing stuff, simply lead and fire like a PT.
When submerged and scope down the boat is invisible. But..if the scope was seen by a plane or ship, and they fire or drop a bomb on it..even if hidden, it would take damage.
For sensors, the sub would see all enemy fleets within its radar range. As well as all air contacts.
The sub would be well perked so there arent a million out there. They could spawn from ports or fleets.
The surface fleet could contact the enemy sub...and provide a sub warning, only if fired upon by the sub.