My wife and I do a lot of sea kayaking, and it is often very rainy in our area. We use proper roll top dry bags. More expensive than the garbage bag method, but also much more durable.

You can get roll top bags in various sizes. Because kayaks are such an odd shape and often have small hatches, we mostly use the smaller sizes of dry bags, and one uses them to fill the compartment like building blocks. We even have special compression ones for our sleeping bags (quite expensive, but worth in in my opinion. I don't usually use hard dry boxes since they don't pack as efficiently into kayak hatches, and dry bags are as reliable (if not more so) for keeping stuff dry.

For canoeing I would expect you could do fine with a couple of larger roll top bags, rather than many small ones like we use.