You should wear a personal flotation device when canoeing, there's a strong correlation between fatalities and a lack of them. The long, politically-correct sounding name does refer to something different than a safety vest. Safety vests are the bulky devices you'll see on big ships. For a lot more information on life jackets, personal flotation devices, and why you should use them, check out Wear a Life Jacket and Transport Canada's page dedicated to them..
I used to use a fancy flotation device that automatically inflates with carbon dioxide when it's submerged in water. The only reason to use this kind of PFD is fashion, it looks a lot cooler than the puffy foam ones everyone wears. For some people, not looking like a dweeb is enough encouragement for them to put their flotation device on. I'm told these are not approved by the Canadian Coast Guard unless you are wearing it all the time, though. It's tricky to find exact details on that. They are not approved for jet skis and other "personal watercraft".
I use standard, dorky PFDs now. I think they are safer since they are always flotation devices, they don't have to inflate to save your life.
Wear your PFD all of the time. I had to fall out of a canoe to realize how important it is!
A whistle isn't just a safety thing, it's useful for communicating with your group, too. Sometimes your buddies can get too far ahead or you want to split up to find the best camping site, and without a whistle that's tough.
You can buy a whistle for about the price of a cup of coffee and most will do just fine, but don't go too cheap! There are electronic whistles and military whistles that work when wet so well that they work underwater, but a basic whistle will do so long as it's not a total piece of junk. Be careful though, worthless whistles certainly exist and will screw up when wet or just in general.
If you're stuck in the wilderness without a whistle, you might be able to make use of an emergency substitute!
You should have a bailer, rope, emergency food, and a flashlight fixed to your canoe at all times. The bailer is useful after you capsize, because you are certainly going to have a bit of water left in the canoe when you recover. It's also useful if you have a small puncture in your fibreglass or kevlar canoe. The flashlight will be handy if night sneaks up on you, because unless the moon is out you are going to have a hell of a time getting to shore safely.