There are lots of GPS units on the market these days and they aren't too expensive. They mostly work the same way, and if you learn how to use one you can quickly learn another.
The first thing to do with your GPS unit is to make sure you have the right maps for the area you'll be canoeing in. By default you might only have maps for urban areas. You might have to pay for the extended maps! Make sure you get maps of high resolution! A 1:50,000 scale map is good. That scale means that a distance of 1 cm on the map is 500 meters in real life. If you get a lower scale value like 1:100,000 1 cm on the map is 1 km in real life. The problem with those scales is you'll miss out on features like small islands which will be very confusing when you're trying to navigate.
Next you'll want to set waypoints. These are the spots on the GPS map that you're going to point your canoe towards. Set them for points on islands, keeping in mind you shouldn't go too far from the shore.