Yes - I suspect this is because the location is pulled from Bing and is the location of the land terminal, not the runway, though these (the symbols) can still be inland by 100’s of feet from the shore.
You are correct. I spawned the C172 (Amphibious!) at CAM8. In cockpit, I see 320ft:
LittleNavMap shows 270ft:
I’ve not tried the other airports you mentioned, but see no reason not to see the same. There are a number of existing bug topics about incorrect water elevation in Canada, e.g.:
but you could add the airports you’ve listed to the MSFS 2024 Airport / Helipad Issue Tracker if they have not already been reported there.