[devilbunny]

Without oxygen, any decay is going to be incredibly slow. They're still pulling usable timber from trees that sank in rivers after being cut in the southeastern US 200 years ago. It's not cheap, but if you want some real old-growth oak, it's a viable source. And that's not a completely anaerobic environment.

[williamdclt]

200y is not really an incredibly long time at all

[devilbunny]

No, it's not, but that's without any noticeable decay. A tree that's been felled in that environment and left above ground will rot very quickly.