You should always wear a PFD or buoyancy aid when canoeing or kayaking. Making sure your buoyancy aid is a good fit is important too. Here's how:
Three step fit
Step 1 Loosen all straps and buckles.
Step 2 Put on the PFD and do up any front buckles or front zips (not including chest harnesses).
Step 3 Tighten all straps, starting from the waist and working your way upwards to the shoulder straps. Finish by sitting down and checking shoulder straps.