Waterlogged timbers that have been submerged for a long time typically have a case of rot that is arrested by the oxygen-excluding action of the water. Expose them to air, and they just crumble away,
A year ago, when the water got abnormally low, the wreck of a ferryboat was discovered in the Sulphur River here in East Texas. We decided our best course of action was to leave it submerged until such time as we have funds and facilities available to do proper conservation of it. Even so, underwater archaeologists who checked on it this year report that the parts that were out of the water (exposed to air) last year have essentially disappeared.
Some conservation techniques include soaking the timbers in glycol and waxes -- to displace the water, while simultaneously excluding air. That requires a big conservation lab and big tanks...
Watch for a FReepmail...
Thanks to both of you for your data-rich answers. Makes total sense...