Hmm. Aren’t the Great Lakes in North America connected to each other and then to the St. Lawrence River via Lake Ontario, which eventually leads to the Atlantic Ocean? Maybe the map should include those large bodies of water too.
There are fish that go from saltwater to freshwater or vice versa. They're called anadromous fish. Salmon are one species like that. So you'll have to include every river as well.
Lampreys are the greatest explorers among saltwater fish. They've mapped most of deepest darkest Greatlakesica, but most don't make it back out to report their findings.
They are. Also you can get to several of the Rockies, even past them,
e.g. The Fraser River will take you to Yellowhead Lake by Jasper Alberta.
Mackenzie River in the Northwest Territories will get you to Athebasca Lake, which also drains into Hudsons Bay.
For that matter, most of the Canadian Shield is linked by rivers and creeks to Hudson's Bay. Which is how that company used to be one of the largest "countries" on the planet. Their deed was to all land all water which flowed into the bay. The English King who did that was an idiot.