As it says on the tin.

Using origins: 0.7.3.5
MC: 1.16.5
Forge: 36.1.24

Underwater mining is still acting slow, even when shifting down onto the block so I'm not floating up. Thankfully I can resort to magical mining with ars noveau. I have since removed better diving so it shouldn't be related to my previous issue. Mods are... still numerous. But I'm not entirely sure what could be denying it if anything.

Just saw that you wanted more info on what mod interactions break Aqua Affinity, one mod that causes this is Project MMO. I already mentioned this on their discord but got no answer, but afaik their dev is pretty busy at the moment. I played around with the bugfix posted above, but I don't know Java and couldn't get it (or something slightly altered) to work. I believe the problem is similar but not quite the same (but again, don't know Java). Even if it's something on PMMO's side I'd love something like the bugfix you posted if that is possible.

I also had this problem with the mod Axes Are Weapons. Is this on their end or this end?

https://www.curseforge.com/minecraft/mc-mods/axes-are-weapons

Also having this exact same issue (same mod and all; project mmo) here in the future of 2024, was there ever any fix?

As far as I am aware this is a mod interaction issue as it doesn't occur in my dev environment.

darn it, I had a feeling. Guess I'll have to take a poke around... :V

oh! shoot, I never came back to report on what fixed this. it was actually a mod called annointed items because of some kind of implementation with it's "mining rate" annointment that can be applied to equipment. Despite never having been annointed yet it would affect all forms of non-vanilla aqua affinity and such. Mod author also doesn't have an issues page or github OR comments enabled and insists on wanting to be direct contacted through instagram for some reason or another. So I can't exactly report bugs to them easily without going well out of my way to do so. :V
https://www.curseforge.com/minecraft/mc-mods/anointed-items

While the fix is trivial on their end (Changing a method call), I don't have high hopes that they will implement it in a timely manner.
If you want to fix them manually, just drop the files in the following zip file in either origins or annointed items META-INF directory in the jar file. This will fix the bug. (It's a simple coremod that replaces a method call)
Bugfix.zip

intriguing. I don't have the mod installed anymore, but if thats a fix, hopefully they can be contacted to fix it and upload it, so that eventually I can include it in the pack when it's finished eventually.