Subs should be glass cannons, great damage delivery, but easily destroyed by a single torpedo of any kind and damaged sufficently to prvent diving once hit by any shell.

In the process of making subs as vulnerbale as they should be, provide subs greater protection through the same means used in real life...by being submerged, or in our case, prolonging the current dive times sigificantly.

As a counter to the thinner skin of subs, you can reduce depth-charge and Hedgehog damage, but increase their critical chance.  So subs can have more survivabilty against the individual DC or HH, but with an increased chance that at least one DC or HH might crit the sub suficiently to possibly force the sub to the surface.

Other comprimises for longer submerged times could be longer torpedo reloads.  Longer time to excute a dive to submerge.

I think this would make subs less spammy, more stealthy and thoughtful and remove the personally annoying issue of them absorbing damage like Battle Cruisers...yeah, Im talking about those subs with over 20k durability...thats ludicrous(no sube should ever exceed the durability of a FF).

Additional changes coul dbe giving low tier subs more love by not gimping thier submerged times so much, higher tier subs generally already have the inherit advantage in speed, torpedo tubes and torpedo storage, no need to make the low tier subs less submersible.  In fact, one benefit of higher tier subs could be the addition of snorkels for nearly unlimited submerged time(assuming no hull damage)...though at the expense of possible slower submerged speeds(to prevent water being drawn in at high speeds by the snorkel's bow wave)and higher/farther detection by the enemy when in use.


Thats my 2 cents.