Nice to see an awesome build by a fellow Canuck! Well done sir!
Regarding water damage to the plywood:
1. I think from your pics of the wall framing that the bottom plate is pressure treated. Did they put a sill plate gasket under the bottom plate between it and the concrete, like this:
https://www.rona.ca/en/gasket-sill-plate-gasket-405-0917003
2. Is the plywood resting on the concrete?
If the plywood is touching the concrete your worry is well founded. You could have put a strip of PWF plywood, a 5/4 or 2x4 pressure treated lumber horizontally on the bottom and rest the sanded plywood on top. I would definitely caulk the bottom plate.