I waited for 24h after finishing the quest, but the mine does not drain. I already did a lot of research on this, as it seems to be a common issue, but also one that has never been clearly resolved. There's a lot of buts, maybes and conflicting information on this out there and I would like to know a few things for sure to narrow this down.

1. Do you need a checkmark next to the upper "Talk to Mathis" for this quest to work? The quest is in the finished portion of my quest log, but it does not have this checkmark, even though I got it when talking to, it didn't carry ovber into my quest log.

2. What EXACTLY triggers the mine to drain? There's so much conflicting information on this out there, does anyone know for sure?

3. Any other help resolving this issue would be much appreciated. Any idea what causes this to bug out and how it can be resolved? I followed all steps of the quest correctly and got checkmarks for each of them on screen. My best guess is the checkmark for the second "Talk to Sully" not carrying over into the quest log is what's causing the problem here.

Can anyone even see this? I can't find this post on the forums...

It's been a while, but you might have to travel away form the location and travel back after 24 hours.

I did that, went to Sanctuary, rested for 72h, went back to the mine. Still full of water.

You drain it by activating the pump switch near Sully, then 2 mirelurks attack.
Did you do that?
If Sully won't talk to you after unblocking the pipes, try opening the console, clicking on Sully and type disable.
Then type enable and exit the console.
Then see if he then tells you , you should be the one to hit the switch.

As I already stated in my opening post, I finished the quest. I did all the steps, it's marked as finished in my quest log. From other posts I've seen, I'm pretty sure that wiki is just wrong (again) about the mine draining after 24h. Noone seems to know what exactly triggers it, but almost everyone says it takes longer than 24h or even 72h.

Sometimes it's not just a factor of time, but also space. If you're in or near the cell in question, you can wait as long as you want, nothing will happen. Sanctuary is quite close to the quarry.
Sometimes skipping time by waiting or sleeping won't be enough either, you have to pass the needed time in game, that is playing somewhere far from said cell.
My best advice is to go to Diamond City for example, wait 72 hours, then return, or if that doesn't help, just play the game normally, leave the quarry and the area alone, then return at a later date.

Well, the mine finally drained for me, even though I didn't have the checkmarkm next to the upper "Talk to Sully". So that appears to be just a minor display bug.

Took a long time though, several days of actual play, not resting. I'm not sure, probably 2-3. So don't believe the wiki when it says that it will drain after resting for 24h, the amount of incorrect and misleading information on there is staggering.