[2022] Subs at 10 km vs a light cruiser who wins?

Please post a Replay. It's always best to have a fixed example to look at rather than bounce around in hyptohetical.

I'll look this over soon-ish!

Replay Review using Screenshots for the above Replay:

Your starting position to protect B doesn't give you the ability to hold it. With the island in the way, you won't be able to spot or get resets from this position. It could be easier to head to a flank and use your smoke to hard farm whatever enemies push.

Heading middle in anything easy to spot tends to result in death, so the enemy team is keeping to the flanks. This does place them out of range for much of the opening minutes.

Fast-forward some 5.5 minutes later and you've been able to work on enemies that backed away from A and kill a Seattle that aborted a push on C.
Note: The match has 11 minutes remaining and is very close in points:

Some 30 seconds later, you elect to leave your position and head toward A. It's a 2v2 at C and a friendly is capping. You are about to lose a Vlad at A so moving there to potentially threaten the cap and/or support the Republique is fine.

The Missouri pushing in means Smoke will not assist you, so you drop torps and continue north to put an island between you and the enemy battleship. Not only has the Vald been sunk, but the Repub has been sunk as well.
C is being retaken, likely by the U-190, and many of your team are closing ranks between B and C. It's possible they will work down the Missouri in time.
Note: The game is still close with 9:43 left to play. It's only 140 points difference but losing C will hurt long-term.

With the Missouri almost behind the island, you throw some shells at the it for around 7,000 damage and a Fire. You were dark as soon as the Missouri went behind the island, so nobody was in line-of-sight. A few seconds later, you are detected again after your firing bloom has lapsed. Odds are good that's the Salmon.

You notice this and activate Hydro shortly after:

The Missouri is sunk, but the enemy takes C. The points are still close enough to be recovered with cap pressure as it is now 5v4 and 8:49 remaining on the clock.

The first ping hits your ship and you look over to where the RPF indicator points. Seeing the visual ping helped as well.

A torpedo is spotted by your Hydro, as well as a Ping and the Indicator that shows rough positioning of the enemy Sub. The indicator looks to be a fair distance away, so it can be assumed that the Submarine is on the surface since it saves Depth Timer and allows the Submarine to spot you at Surface Detection ranges. If the Submarine was a Periscope Depth, it could only see out to your Aerial Detection which is only a little larger than you Hydro's range (which can detect Periscope and Surface at full range).
Note: See Mechanical Description after picture below
Note: You only have a Single Ping active (the highlight on your hull is Yellow, not Green. Single Ping Homing loses lock at a fair distance from your ship, so you ideally want to angle around 30 degrees to the Torpedo so you can cut inside of it when it goes dumbfire due to proximity.)

Instead of giving a significant angle, you have remained mostly bow-in. You even used DCP though there was no need to do so. Given that you altered your course before you used DCP, the torpedo altered its aim to your new course before the ping went away.

As the torp corrected before the DCP, it hits.

You are still spotted which means it is likely the Submarine is surface and vulnerable to HE Shells. Any concern about firing should be minimal as you are already detected anyway. You should feel free to use the rapid fire of the Smolensk to spam around the area where the pings are coming from.

The prior ping missed. Another indicator shown and the visible ping as well. You should shoot at the area to damage the sub or drive it underwater.

The prior ping hits, but the torps sail by. They were too close to care about the ping.
The Montana and Musashi are now spotted, you could head off to your right to take A. Smolensk's smoke screen is a great tool.
Your team is pressuring B and two members are looking to push/capture C as well. There are 8 minutes remaining so the 134 point difference isn't that concerning.
Another Visible Ping and another Ping Indicator. No shots are fired. You should be shelling the water to get Damage or Oil Leaks to give you more information, or at least the sub's DCP.

Another Visible Ping and another Ping Indicator. No shots are fired. You should be shelling the water to get Damage or Oil Leaks to give you more information, or at least the sub's DCP.
If the sub goes under the water, you could go dark. Or you could be turning off to take A to give your team points.

The prior ping hits and you open up to side to aid in avoiding the homing torp.

But without getting the torpedo to lead itself off ahead of your ship (so you could cut inside), the torpedo corrects and impacts the ship.
Still, no shots fired at the Ping Source.

Off-angling occurs earlier for the next Torpedo.

But no cutting in results in another hit.

Your Hydro is now down. The Submarine can now safely operate at Periscope Depth while still being able to spot you due to Cruiser Aerial Detection being roughly 6km. It might be easiest to turn away and head to A so that you can cap it.
Your Izumo in B has claimed the cap and C is being contested. There is 7:23 remaining in play and only a 164 point difference. This is not a kill-all scenario.
Another Ping Indicator and Visible Ping, still no firing. This is just attrition as you are losing health to gain ground that doesn't matter. The A cap is the most important place to be for you right now. Especially if you can find a way to put shells down-range onto the Battleships after taking A.

This is part 2 of the Replay Review (too many pictures). Please start with the post above.
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A recent ping has struck on the tip of the bow. This is a good position for you as it causes the "Interaction Point" (where the Torpedo and Ship would collide) to be projected far ahead of your ship. This allows a lot of room for you to cut-inside.

And you did!

Continuing to push with no Hydro is dangerous. The A cap is off to your side and your DCP is ready in case you need to shake off a Ping effect.
C is being contested. B is taken. It's roughly 140 points between teams with 7 minutes to go. This is not a desperate situation the requires running down the submarine.

Another Ping Indicator and Visible Ping. Still no shots at it.

You DCP before seeing an incoming Torpedo. Instinctively you turn to Port.

But the Torpedo was going to miss to Port and will now hit. With no DCP, this could hurt.

Now you have no Hydro, no DCP, and a loss in Engine Power due to Flooding. And low health.
The chase has resulted in attrition and not achieved any points for your team. This chase should been aborted in favor of taking A cap.

An odd torp appears off-track. With health this low you might expect the Submarine to get greedy.

You are not spotted. The Submarine is now at Periscope depth. You are getting close enough that they can no longer be on the Surface, but you can still hit them at Periscope depth. However, that could flare your Aerial Detection and get you re-spotted... perhaps a Smoke screen and last chance attempt to disengage?

You continued onward. A visible ping went underneathe your hull as the submerged sub could not see you anymore.
Then you are lit and a Ping and Indicator appear in front of you. The Sub is committing. You should be firing at the Ping knowing the Sub is surfacing to attack.

The sub is spotted as you two are very close. The first torpedo misses. Others are on their way. Still no firing.

A single round sent off from your guns, but the Submarine dove hard as they were spotted. The torps collide and your game is over.

The score at this point is now 316 apart with 5:47 remaining. That is a tough ask.
If you had capped A two or three minutes ago, the enemy would be some 70-105 points poorer and your team 70-105 points richer. As well, you may have been able to help work on the battleships in the East.