They use channel 16 just like we do, and have basically the same regs also.
The handheld 5w radio will do about 5 to 7 miles over water, holding it standing up in the boat, which is pretty much your line of sight. You are up a river, expect less.
The coast guard has their antenna up a tower, typically 400' up in the USA. I would imagine similar in canada. If you are within 20 to 30 miles, calling the coast guard for help should be no problem.
On a small boat I dont see much point in a fixed radio and antenna, the higher gain will extend range to about 10 to 15 miles, but your line of site still limits your range when your antenna is mounted on a small boat.
VHF will work better out in the wilderness, especially in rough terrain. UHF does better in town, better penetration into things and buildings.
This is not calculations or what I figure, this is what we used to get out of our radios when I lived on the coast.
