For the first ten years I was in the School Club, we ran several Chestnuts and Tremblays. They would start off the trip around 80 pounds, and finish at well over a hundred. The water did indeed come from the inside. Our canoes were well seasoned, and many of the chestnuts and tremblays were not finished on the exterior plank side, so any water that entered the canoe, and there was always lots, was sucked into the planking mostly, but often the ribs too. It would usually take a few days, but by the end of a 14 day trip, there was a very noticeable difference. I believe the canvas would become somewhat sodden too from the inside, as only the exterior is treated.
Paddling in the rain, wet entrances and exits, running lots of big rapids....all of these things contributed to constantly soaking the canoe.