Having played as and against subs, and being a submarine fanatic, I have seen that the submarines are currently implemented poorly. The matches I play as subs are either 150k+ dmg matches where you butcher a helpless enemy team, or frustrating 0 dmg insta-deaths where you get annihilated by swarms of depth charge bombers. The current implementation has serious problems, and much of the community would rather bandwagon on the hate-train against subs opposed to actually offering solutions that satisfy everyone. I intend here to provide a conclusive solution that will satisfy all players, submarine enjoyer and otherwise.
The solution to submarines is as follows:
1: Remove homing torpedoes and depth charge planes.
Reasoning: Homing torpedoes drag the skill floor of submarines way too low, and allow subs to strike targets they definitely shouldn't be. For example, these homing torpedoes can counter DDs/cruisers sitting in smoke, submarines even partially countering DDs is extremely silly. As for depth charge planes; this means that BBs become the primary counters to submarines, I can definitely testify to this from personal experience. The design of submarines should be focused on how they fit into the game as a whole, BBs countering subs better than DDs is insane. DDs should be the primary counter to submarines, more on this later. All classes of ship should have access to simple depth charges, with DDs possessing the best (greatest quantity of depth charges released).
2: Refocus the purpose of submarines to be anti-CV.
Reasoning: CVs are known for playing an extremely safe, passive playstyle with respect to their ships positioning and movement. They currently have no counter, submarines can fill this void. Giving submarines increased damage, even a chance to land citadel torpedoes against CVs ONLY. This increased damage potential will motivate many submarines to seek out CVs. People will often go where their most reliable way to get damage and citadel ribbons is, this is how you can motivate anti-CV play.
3: Allow nuance in play for submarines by changing how sonar works.
The submarine should draw inspiration from the Spy from Team Fortress 2, a backline assassin with a few distinct methods of play, either super stealthy but only able to harass a few (albeit more reliable) backline targets, or super high risk high reward (less stealthy). Here is what I mean, the sonar ping and hydrophone consumable should both be removed in favor of a toggleable sonar. When the sonar is turned off, you are blind, but are far harder to detect, only able to see other submarines at extremely close range (where you can hear their propellers). This allows for a low-risk way for the sub player to sneak past the frontline and sneak to the backline where its intended targets are, CVs and sniper BBs. The alternative would be to turn on the sonar which would reveal the submarine to other submarines underwater from a significant distance (5-10 km depending on concealment). This offers a higher risk, high reward style of play, with the sonar having a long range detection of underwater ships, while a shorter range (~75% of underwater) detection of surface ships. The hydroacoustic search consumable should only find submarines who either have their sonars on, or are extremely close (where the propellers can be heard by the hydroacoustic instruments).
4: Change how submarine spotting works.
When a submarine spots another submarine currently, if both of the subs are near the frontline, they both get descended upon by tons of depth charge bombers and often both die before they can even kill one another. After my suggested changes, the subs will now be able to duel properly if they so choose to by turning on their sonars. However a submarine being a hard to spot machine itself, it being able to spot everything better than a DD makes no sense. So rather instead of a sub being able to spot enemy ships with its sonar, it instead only spots the ships for itself (and possibly for friendly submarines). If the sub is at periscope depth or the surface, the spotting functions as normal, but comes with the downside that it is detectable by nearby surface ships.
5: Revert the dive timer back to the battery.
This is a no-brainer, simply make the sonar consume battery when it is on. This would make submarine duels extremely interesting games of battery management, juking/3D positioning, and leading torpedo shots. I suggest allowing submarine torpedoes to arm much sooner, allowing for closer knife-fights underwater.
6: Make the different depths matter, and partially revert the diving ability.
The 60m depth should make the submarine extremely hard to spot, and reduce/eliminate the damage from depth charges. The downside of this would be purely the agility of the torpedoes themselves. Torpedoes can only rise so quickly, so this would significantly limit the subs ability to deal with close range targets, and without guided torpedoes the long range targets are much harder to hit. This balances itself out. The periscope depth should, like a few submarine updates prior, allow for the recharge of the battery. As for the diving ability, to prevent abuse it is good to leave the surface and the periscope depths as preset depths where you cant feather in between periscope depth and just below it, however below a certain depth the diving/ascent shouldn't be constrained by preset depths, only having a maximum depth which can vary by the submarine (this wont be too much of a problem in submarine duels if my point #8 is added).
7: Reassert the DDs place as the submarine counter.
DDs (and light cruisers to a lesser extent) should possess innate vertical sonar, giving a cylinder of detection directly below itself, perhaps about 200m in radius. This, coupled with my change to spotting, means that if your friendly submarine spots an enemy sub, your DDs will know the general area to start searching and should be able to find the sub quickly. This combined with being able to release a long chain of depth charges should scare many subs away from trying to meddle with the frontline, as usual they must take a high risk for the potential of a high reward.
8: (New mechanic) Allow for torpedoes to be aimed both horizontally AND vertically.
This would simply add another axis to being able to launch torpedoes, this is so sub vs sub fights can allow for leading shots in all 3 axes of movement, since subs can juke by changing their depth. As stated previously torpedoes are only able to ascend/descend so fast, limiting the ability for subs to strike at surface ships from too deep down.
I am convinced that submarines can be a fun addition to the game, where they also improve the health of the game by discouraging unfun playstyles. Should my changes be implemented, CVs will no longer be able to feel "safe" every single match, and be able to just sit in the back and send their planes out. This will force team coordination on the part of the CV (definitely a good thing), as should the CV wander too far away from friendly ships, they will easily fall prey to a sub. As a person who also enjoys CVs, I hate seeing CVs that just sit back and play it super safe all game. CVs should move up and act more as a part of a battlegroup. This to me is the most enjoyable way to play as and against CVs, as it gives a chance for more daring players to attempt to pierce the battlegroup and strike the CV. I believe that subs can definitely function as a counter to the CV/sniper-heavy meta we've been seeing for a while. Subs will make the backline an extremely dangerous place to be, and encourage more active play on all parts.
Thanks for reading my ten thousand leagues of text.