Submarines in naval battles midget for coastal, regular for blue water battles

Why aren’t there any submarine midgets for coastal and regular boat size for blue water battles?
Wouldn’t it make spicyer et more realistic in realistic mode?

Submarine thread if you are interested

But basically : because it takes time to develop, and they are hard to balance (submarines are notoriously slow, and fire slow weapons in the form of torpedoes)

Obviously, we need Surcouf then. Cannons.