In my experience there are but 2 conditions to really worry about that will cause the canoe to fill with water. #1: big water,with large waves caused either by high wind or by crazy motor boats. Waves wash over the canoe, from either end, or wash along the gunwale especially when quartering from the side, thus filling the canoe with water. I was once in a slowly sinking voyageur canoe ("we're going down!") after wind and wash from a large but relatively distant motor boat traveled the length of the gunwale, filling the canoe so that the crew had to abandon ship and swim to the thankfully nearby shore.
I have been sphincter puckered a few times by wind driven waves coming awfully close to gunwale height, but only once thought we were going down.
I had paddled into a site on Grand Lake Matagamon in a small tandem and set up camp. A couple days later I paddled back to the put in, picked up a friend and his gear and we paddled back to camp. The plan was for him to stay a few days and I would paddle him back to the launch and then return to camp for a few more days solo.
He stayed an extra day. I did not feel like paddling out, in and back out in 24 hours time, so we loaded all of the gear in the canoe for a single trip out. Everything was fine until we rounded a point and got into the wind. A tail wind, manageable at first, but as the fetch increased the waves started inching up towards the gunwales of the overload canoe.
And then pouring over top the gunwales, a couple gallons at a gulp, which was not helping our lack of freeboard. We had one chance at a tiny rock spit of an island and just barely made it, beyond that was a lot of open lake stretching ahead.
Nothing was tied in, and I am not sure it would have mattered. We could not have managed a reentry in those conditions, and if we sank with the inevitable waterlogged canoe slow roll over we were probably not swimming anything except ourselves to shore.
There were two lessons learned there. Lesson 1, as soon as I saw the conditions on the lake I should have turned us around, either back to camp or into the waves away from the put in, towards the protection of the far shore. Once out in it there was no turning around. Route planning matters.
Lesson 2, we were in a none too deep 15 foot tandem, which was inadequate for two guys with a hefty gear load. Too much freeboard is a pain in the wind, but too little is worse.
#2: most times in normal circumstances the canoe does not tip far enough for water to flow in over the gunwale, rather a paddler does not follow the rule to keep head inside the gunwale. Head outside the gunwale and the paddler simply loses balance and falls out, leaving the canoe high and dry until the paddler grabs the gunwale on the way out and causes tipping so that water floods over the gunwale. I've directly experienced that phenom too. Caution, try not to do that with an audience.
Either way you end up with a boat load of water.
More times than I care to admit, and always with an audience. Sometimes taking the canoe over with me, but as often hull sipping just a few gallons of water as I went overboard.
That has never occurred in any challenge that required attention, rounding a tight strainer, making a move in rapids or catching an eddy. The canoe ejections have always been in relatively benign conditions, which led me to become overly inattentive.
There is a lesson there too, about paying attention. And a conundrum, I am more like to tie in for challenging conditions and less likely for the benign. Which, it seems, is when I swim.