Where in Canada can you hike under frozen water?

I think you have been reading a few too many children's books. Your question probably stems from a book called "The Very Last First Time" about a young Inuit girl who goes hunting for mussels under the frozen ice near her home while the tide is out, and she begins to day dream and marvel about the shapes and sights beneath the ice, forgetting that the tide is coming back in.

There are several problems with that story.

First, in most places in Canada, the tides are not so large that you could actually fit a person under the ice. Perhaps in the Bay of Fundy, that sees the highest change in tidal levels, but otherwise it may only drop a foot or two.

Second, if the the levels for the tide are so different, when ice forms at low tide or at high tide, they may end up crushing each other, with low tide ice floating up to high tide level, and high tide cracking and falling down to low tide levels because of the lack of support underneath, much like we see in puddles on the road when the water drains out and air gets in underneath.

So while it may make for an interesting story and cause young kids to ask interesting questions, I don't think there is any truth to the story.

There may be places in Canada where you can hike under a frozen waterfall, if the back of the waterfall is pushed in and forms a narrow passageway back there. But I've never heard of anyone walking under a frozen lake after the water drained away, even during tides.

ive lived in yukon all my life and never heard of it

Nowhere! Never heard of it.