Sub's role to me seems to be a bit at odds with how Navyfield works as a whole.

In Navyfield, forces are pitted against each other in a large scale deathmatch.

Submarines are stealth ships meant to pick off surface ships, such as carriers and occasionally the odd battleship.

Teams are pitted against each other in a large rectangle head to head, whereas in order to play the stealth role in this situation flanking is required. This is not possible because of the map size in combination with how spawns work. The directions and places that submarines go can be simplified to  north, south, or mid. This makes anti-sub countermeasures extremely effective.

Now, since submarines cannot play the stealth card, what else can they do?

They do what they do in current NF, which is to hide from normal damage and act as BB killers.

Ships of importance besides BBs cannot be sniped by Subs because CVs can hide so far in the back of the fleet that submarines cannot reach them, and because things that subs would be going after in traditional war, that is, supply ships and transport ships, do not exist in standard Navyfield.

The only role that Subs have right now is to act as a glorified torp launching BB counter. There is no fix to the submarine ship itself that will fix it.

I have seen a few suggestions and here is why they are unfixable.
-Reduce torp damage, increase air time
Basically an increase in air time would be a bad thing in general mostly because with the current air recharge times submarines would be untouchable. A level of vulnerability to all ship classes must be present in order to provide good game balance.

-But we can just reduce air reload!
Then submarines are reduced to one shot glass cannon. One of the best tactics I have seen is to pop a smoke in the middle and regain some air. Furthermore, it does not address the main concern with submarine gameplay. Submarines should not be engaging Battleships.
Another thing to keep in mind is that submarines need the high torp damage right now in order to do any damage since bulge negates the first wave on almost every bb2+

-Buff asw
The day asw gets buffed is the day every sub quits ever. asw is simply too easy to play, and buffs to it in its current state will not work.

In my mind it would be nice if the submarine was a counter to carriers. In turn ships of the line (FF-BB) would counter subs, and carriers are basically BB player's lifeline. A good Sub player could change the tides of battle by killing a key CV or in harbor assault, destroying apas.

The spawns are the main problem with submarines. In order to maintain the element of surprise, the angle of attack must be random.

A spawn that features head to head fighting between two teams in Navyfield will never allow for balanced submarines, or for submarines to do the role that they were intended for.

Therefore I suggest randomized spawns in a large playing area along with according buffs so that submarines and scouting will be useful.

In such an environment scouts are required to find the enemy in order to prevent flanking and backstabs and therefore swift destruction.

Submarines could provide some of all of this. They could act as underwater scouts, they would perform flanking maneuvers impeccably and therefore function in their role as the counter to CVs, which, besides other cvs, would otherwise have no counters.

In an environment where subs could affect the outcome of the game as much as any bb or cv, asw would therefore need to be very alert. The thing about random spawns in combination with a much larger map is that holes in defense are much larger, and therefore asw loses it's opness.

Submarines need a change in the mechanics of how they play, not in anything related to the actual ship (although changes in the hull would need to be added in order to balance it).

tl dr; random spawns would make for much better submarine and submarine counter gameplay.