Do you keep your phone in a case? Is the headphone jack opening obstructed in any way by the case? Typically most cases are made to allow that opening lots of open space but it's just something to check. And this issue is more of a problem when the headphone plug end is one of those with a right-angle involved as opposed to when the plug end is a straight-through type.

The whole premise might be easier to understand is if you visualize what happens when you insert the headphone/earbud port into the headphone jack of your phone. If you look closely to the jack, you'll see at least three metal sections, each separated by an insulator. Those three metal sections correspond with the ground, left channel, and right channels of a typical audio signal. In your phone at the bottom of that headphone jack opening, there's a mechanical switch with three metal contacts that directly match up those three metal contacts on the plug. When the plug is fully inserted the metal contacts on both are in contact with each other, so at that point whatever audio signal your phone is making is now going through to your headphone/earbud. But it is important the plug is fully in position, that's why I brought up the possibility of a case being an issue. (Bluetooth headphones rely upon audio signals going through the Bluetooth radio chip in your phone, the headphone jack not being involved in any way.) Of course, you might not use a case or it might not be an issue at all, it's just a suggestion based on supposition.
You did make a reference to water damage, which 'could' be part of the problem. That headphone switch mechanism is one of the few components in your phone that has moving parts, a spring-loaded metal contact that also functions as a switch where an audio signal gets re-directed to the headphone jack away from the internal speaker(s). So if in the accident that was part of the water damage left behind residue on the jack's metal contacts (in your instance, the right channel), that might be tied to your sound problem.
The thing is it's really difficult to actually physically clean any of those metal contacts. That headphone jack opening is pretty tight, and even if you do a tear-down of your phone to expose its internal components, the headphone jack is typically an enclosed piece. Replacing the headphone jack is also a very conditional problem as only some phones have modular, easily accessible jack. With most phones, its soldered directly to the logic board so 'fixing' a bad headphone jack might involve replacing the entire logic board. Something of a drastic measure.
You might want to try getting an aerosol can of electrical contact cleaner. It's a low residue cleaner for switch contacts, and keep in mind it's also a toxic stew of chemicals with really long, multi-syllable names so use it very sparingly and in a place with good ventilation (one quick squirt is adequate.) But don't get your hopes up too much, it's one thing to clear away minor corrosion, another matter that involves something like a some kind of sticky substance due to the water damage incident.
If you opt to try and use something like a toothpick to dislodge a suspected obstruction in the bottom of the headphone jack, do so very, very, very carefully. If you inadvertently mess up that switch mechanism it might jam itself so the audio signal is always going to the internal speaker or to the headphones.

Worst case scenario -- just rely on Bluetooth headphones. Or go into your phone's Settings >> Audio menu and select mono instead of stereo so the audio signals for the left and right channels get combined into one channel.

 

As mentioned above, switching the phone to mono is a good idea.
This will allow you to see if both sides come on, in which case the problem is not the jack itself, but something else.
Switching the device to mono forces both channels to come out of both sides at the same time.
So if the jack itself is bad, then the side that is not working will continue to not work, because the connection is just not there.

Switching the device may be as described above, or it could be {as mine is}...

Settings
Accessibility
Mono audio

Now on the other hand, if the 'bad' side starts to work when the device is switched to mono, then there is another issue- possibly the app you are listening to, or some other audio software in the device.

 

So to be clear, the right channel of the headphones comes on when the device is switched to mono, correct?

And this problem exists with any set of headphones plugged in, correct?

If both of the above statements are correct, is the problem occuring with any app that you play audio with?

Here are a couple of apps that I use all the time.
I would like for you to try them and tell me what, if any, difference you have with this issue.

First, they are apps that I use, so I can help you navigate them quite easily if you have any problems with them.

Secondly, they are pretty cool apps that function well.

Be sure to switch the device back to stereo mode first.

The first app is called RadioDroid, and it will allow you to listen to worldwide broadcasts of radio stations that also stream their feeds online.

Simply start the app, enter the call sign of your favorite radio station, and listen to the feed.

Take note of whether both channels are working.

https://f-droid.org/en/packages/net.programmierecke.radiodroid2/

The next app is a music player.
The link is for an old version, from before the app had ads.

It is called BlackPlayer, and has some cool features.
One of these features, the one that we are wanting to use for this experiment, is a volume balance control between the left and right channels.

Start the app, and give it a bit to find your audio files.

https://blackplayer-music-player.en.uptodown.com/android/download/345945

After it starts, go ahead and select some music to play.

Swipe to the right, and select:
Settings
Audio
Select Equalizer
BlackPlayer Equalizer

Now go back to the app's homepage, swipe right again, and select Equalizer.

Don't turn the switch 'on' yet. Leave that for later fun with this app.
For now, swipe left, and that will reveal a secondary effects page.

Here at the top are the left and right balance controls.
These are actually pre-amp volumes, which are separate from the main volume control that you are familiar with.

The default levels are 100%.

So, if only one channel is working, turn that channel all the way down.
Turn the main volume {the familiar volume control} up.
Is there any sound at all?

At the very bottom of this page in the app is a volume boost control.
CAUTIOUSLY adjust this upwards, listening for any increase in volume, anything at all.
I say cautiously emphatically for two main reasons-- the volume could suddenly come on at a very loud level, and also, if you hear any kind of scratchy sound or distorted souund at all- at any volume, bring the booster volume down immediately.

Please get back with me with your results.

 

Well the whole problem appears to be when you plug a set of wired headphones in you can hear sounds in the left channel but the right channel is silent. Again, try to visualize what happens when you plug your headphones into your phone.
Plug end (a typical plug end of the set of headphones/earbuds, and keep in mind the sequence -- ground, right channel, left channel is an industry standard and does not vary):

Note position of metal connector for the right channel audio signal.
Socket (essentially the component as the bottom of the headphone jack opening on your phone):

So when you plug in your headphones, you're creating a circuit where that the left channel metal contact in the plug has to be in direct contact with the left channel metal contact in the socket, and the same applies to the right channel metal contacts, and grounds. In your instance something is apparently messed up with the right channel contact of the socket, since the same situation is apparently occurring with any headphone you plug in. Again, it could be something you'll eventually be able to clean but that vague reference to water damage could also be a bigger problem -- i.e. rust on that right channel metal contact.
So try to clean up that headphone jack but be very, very careful. Or think about using Bluetooth more.

 

The only way to know for sure is to experiment.
I gave you the tools to give us at least one answer to the questions.

By using RadioDroid, you will see if both channels work correctly-- I am presming that this app is new to you and that you have not changed any of the app settings.
This eliminates the question of whether or not it is one of your apps causing this.

The other app, BlackPlayer, is not only an excellent music app, it also gives us the oportunity to control the audio channels separately, and to boost the volume as well.
This can give us further answers-- is the channel just more quiet than the other?
Is the bad channel working at all?

Also, you did not answer my question, as to if both channels work when the device is in mono mode.

This is very important, because if both channels work in mono, then it is NOT the jack.
This is a process of elimination, and elimination of the headphones themselves and the jack are priority one.

After that, a simple test of a stereo signal from a live feed that has not been modified by app settings is in order, to see if the problem really exists gobally-- and not just with some particular app that we have installed.

Then we can use the other app to actually separate the volumes and then boost the side that is not working-- to see if anything at all is coming out of the 'bad' channel.

At this point, at the best you may discover that the problem is just a setting in some app, or even a glitch in a system app-- most likely in my experience a system app called MusicFX.
Sometimes I mustforcestop this little pest of an app, or even restart the device because this or some other system app is jammed up.

At the worst, you now have two more rather cool apps to add to the new device you will be aquiring if you want to listen to stereo audio.