Breaking tree logs underwater with an axe is inconsistent in breaking speed.
Breaking one log on top of any non-log block has the same speed as above ground (blockBreaking = true), but breaking one log that is below or on top of another log takes around 21 seconds.
Demonstration video: https://streamable.com/hezcg
I figured this was because of the Tree Chopper mod, and it seems to be the case.
I kinda must use that mod because Ore Excavation is unbalanced (excavation speed is independent of tool efficiency and block resistance) and its creator is not concerned with that fact, preferring to tell its users to change the mod themselves. I don't use java so I'm not sure I could do that, so it'd be great if this issue were resolved.

I see. I'm using 1.3.90 of Better Diving and 1.2.4 of Tree Chopper and 1.12.2 of Minecraft.

You can also use Better Diving v1.3.91. There i set the event priority of my break speed event to LOW. That doesn't really fix the problematic code of the Tree Chopper mod but it should still work. So it's more a workaround fix.

Which version of Better Diving are you using?
Also it's a bug in the Tree Chopper mod. The author of the Tree Chopper mod uses the original break speed as the basis for his calculations in the break speed event. But he should use the new break speed as the basis to prevent something like this to happen.