I don't know how being Fully submerged would affect the logs.
I grew up on the Columbia River, near Portland, OR, my Family had a Boathouse (floating home) at Big Eddy Marina, the float was built on very Large D Fir logs, it was more than 30 years old when my Dad sold the place
, the logs were still SOLID at that point, there were plenty of places much older than our's that still had solid float logs.
It was considered very important to have the logs floating at the right level, about 1/3 of the log Faces above water, for the "Health" of the Float, it was believed that too much, or even worse, too little face exposed above water would be detrimental to the lifespan of the float logs.
The float height would be adjusted by adding either 55 Gallon Drums Or Styrofoam "Barrels" smaller floats made of old tires with a foam center would also be used.
Adding or moving Appliances or Heavy furniture could effect the "Trim" of a house and require strategic placement of foam or barrels to level up a house again.
The house floats generally had the logs spaced close enough together that the flotation barrels would nestle between them, and once placed their buoyancy would keep them in place.
Placing flotation was done with a special floating rack that would hold the barrel/foam at the surface under it at one end, then the submerged end would be placed where the flotation was needed, and the Barrel would be winched down the rack and when it reached the end would float up into place.
Placing flotation was a relatively simple matter, but removing it was a whole different story, sometimes, if possible weight would be "Creatively Placed" rather than trying to remove in place flotation, that was one of the advantages of 55 gallon drums, they could be punctured and sunk if need be, and back in the 70's that wasn't the issue it would be today
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Most owners paid close attention to floating height of their logs, and I knew of many floats near the half century mark that still had sound logs, but I don't know what a Century of Full submergance would do to a Big D Fir log, but it still could be a very Viable log.
Having that boathouse for a weekend retreat afforded my Sisters and I a childhood that I wouldn't trade for any other. I wish that I had more pictures of the place, it was destroyed by a Fire many years ago, after my Father had sold it, now it only remains in our Hearts and Memories, but What Memories we have from those days
Doug