Choosing a flotation device or PFD.
Safety on the water comes from equipment and behavior. If you've ever been on a boat, someone should have mentioned lifejackets or PFDs. The terms may be used interchangeably, but they do describe equipment with different characteristics.
Lifejackets are devices intended to roll an unconscious person faceup in the water and have high visibility colors and features. They tend to be bulky and are recommended for younger paddlers, nonswimmers, or rougher conditions where rescue might not be immediate. Class I PFDs also fit this description.
Personal Flotation Devices (PFDs) are made in a wider range of colors and designs, since they are intended simply to bring you back to the surface quickly when you fall in. This more general-use PFD (Class II or III) will be built with a specific price, user, or features in mind. The most recognizable purpose-built style is probably the Fisherman, with many pockets, attachment points, and dull colors. Kids and dogs have their own models as well, usually with a prominent handle for retrieval. It can be tricky to sort out the other styles if you don't have experience wearing a PFD for an extended time or haven't decided what kind of boat you'll be in.
Some key factors to consider:
Fit: Chest size is the primary sizing consideration for a suitable fit, not weight. Most flotation devices will have a weight capacity to handle all but the largest users. Since the human body is about 80% water and thus is naturally buoyant, and most people have about 15% fat which floats, the buoyant material in the PFD need only lift that percentage of you that doesn't float, which by this math means that for a 200 pound person ((80% water= 160lbs buoyancy)+(15% fat=30lbs buoyancy)) is only about 10 pounds. The ratings statement printed on Coast Guard approved adult PFDs will indicate that it is suitable for individuals 'over 90 pounds' because as long as the device has a capacity to lift 20 pounds, it will hold a pretty big paddler without much effort. So don't worry about if it's going to work, just make sure that you're wearing it, or all that math is in vain.
Weather: Summer on the water means sun and sweat, and so your PFD should offer as much ventilation as possible. Winter conditions might mean your chest and arms gain four inches because of thermal layers, fleece, and a jacket over top. The PFD shouldn't inhibit range of motion or be uncomfortable.
Function: Most designs for paddling will be shorter to allow freedom of movement at the waist and perhaps accommodate a spray skirt. This means the flotation material will feel thicker than a design for just sitting in a power launch or riding water skis, because both have the same amount of fill. Other paddle-specifc features will include mesh areas on the back anticipating a backrest and perhaps lash patches to clip on a knife or tool.
It may be tempting to outfit your canoe or kayak with a floating cushion or throwable (Type IV) PFD, but in most non-powered craft, you won't have the space or time to use these, and even with a good throw, the person needs to see and grab it.
Self-inflating (Type V) PFDs may seem like a low-bulk option, but keep in mind that they must be worn at all times to be effective, and require regular inspection and maintenance to function. A budget PFD vest will work perfectly worn or thrown, even after a year in the gear bag- your fancy inflatable might not.
Whichever you choose, make sure it is Coast Guard approved, have enough for everyone who will be in the boat and make sure they are worn while sharing the channel with powered vessels and readily available in calmer sections.