Human bodies are adapted for life in the tropical, terrestrial environment of east Africa, not for immersion in cold water. If someone starts yapping at you about the "mammalian diving reflex," you should probably cram a root vegetable down their throat and find someone else to talk to! That reflex, if it does exist, will be of little use to you while swimming in 40° to 50° F water.
At almost all times, kayaking in Washington requires respect for the effects of cold water. Lake Washington is quite cold most of the year (near freezing in winter) and only really warms up some years in mid summer. Virtually all whitewater rivers are dangerously cold, due to being fed by snowmelt and/or high altitude rain run-off. Except for the most protected bays during hot weather, all salt water is cold during the entire year, regardless of the weather.
Do not be reassured by information that suggests that people can survive in 40° F water for 1 or more hours. Tables of "survival time" are approximations for persons sitting motionless in test tanks or in swamped liferafts. Swimming or otherwise moving will cause you to lose heat more quickly. Cold water can quickly sap your strength and numb your hands, making seemingly easy swimming or rescue tasks impossible. Ironically, an endurance athlete's build (thin, low body fat) will lead to the fastest cooling, but anyone will chill quickly and must get out of cold water as soon as possible. River boaters will usually be towed to shore immediately by other boaters. During a deep-water rescue, try to keep your hands and forearms out of the water as much as possible, so they do not become useless with cold, and try to re-enter the boat before you pump the water out of it. Do not leave your boat to swim to shore unless you are sure you have no other options. Without a PFD, the danger of abandoning the boat is much greater, so a PFD must always be worn and properly buckled and/or zipped.
The effects of cold water are not limited to hypothermia. During the first few moments of a cold-water swim, simply breathing normally is nearly impossible. Instant drowning is not unknown, due to gasping with shock (and thus inhaling solid water) when the head and neck suddenly enter cold water. This is rare, but anyone will find that they breath in a shallow, ineffective manner after first entering the water. Just the surprise of cold on your head and neck can be disorienting and cripple your ability to remain calm and roll up. Fortunately, these short-term effects are reduced somewhat by previous exposure to cold or even cool water and this experience lasts for weeks or months, so frequent, carefully controlled swims or rolls in the coldest convenient conditions are desirable.
Between the instantaneous problem of the gasp reflex and true hypothermia lies the danger of "swim failure." Simply put, take your normal swimming ability, and divide it by 2 for every 10° F that the water is below 70°. This is one reason you must ALWAYS wear your PFD properly fastened at all times. You cannot expect to put it on after capsizing, and without its flotation, you are likely to drown quickly while swimming even if you are a good swimmer, especially if there is enough wind to cause waves that will tend to wash over your head as you try to breath. In February 2011, a kayaker died in Lake Washington after a capsize near shore in which he was apparently not wearing a PFD, even though a witness called police.
Only a wetsuit or drysuit provides significant protection from the effects of cold water while swimming. A common misconception is that wool or synthetics such as polypro/polarfleece will insulate someone in the water, due to the popular idea that "they keep you warm when wet." This only means that they absorb little or no water, and can insulate somewhat when DAMP after you get out of the water, unlike death-sponge cotton. Polarfleece alone WILL NOT keep you warm while you are in the water!
Therefore, river boaters should wear wetsuits, and usually drytops, at all times. Sea kayakers should wear wetsuits and maybe paddling jackets under most conditions, except in very protected areas in warm weather (most SK-II trips and some SK-III trips). Lake paddlers should wear wetsuits except in summer or early fall quarter, unless remaining VERY near shore (roughly within spitting distance). If you get hot, splashing water on your head, arms, torso or even legs will solve the problem, so you should dress for the temperature of the water, not the air.
Wetsuits work by accepting that the intrusion of water is inevitable, but that only the minimum mass of water should be allowed in. That small amount of water can be warmed by the skin with acceptable heat loss, if it is not flushed away and replaced by more cold water. The neoprene then insulates the warmed water and skin somewhat against further heat loss. A 3 mm "farmer john" wetsuit and dry top are not really adequate for long swims in our salt water (scuba divers wear dry suits or full 7 mm wetsuits and still get cold), and a swim in a 40° river is downright painful, but they are still a big improvement over any other type of clothing.
Proper wetsuit fit is important. Suits are labeled with size and intended gender of the wearer. Please try on club wetsuits ASAP and remember what sizes fit you well (more than one size or even gender may be acceptable, since neoprene is pretty stretchy, particularly the newer suits). Ideally the suit should press snugly but comfortably against the skin everywhere. Large wrinkles or empty spaces will fill with more cold water than is necessary. Too loose a suit or wearing thick clothing under a suit will allow cold water to flush in and out repeatedly, carrying away heat, particularly when swimming or moving during rescue. Too tight a suit will be uncomfortable and less likely to be worn, or may cause hands or feet to fall asleep, which makes a capsize MUCH more likely. Ideally, you should wear minimal clothing under the wetsuit (e.g. swimsuit, bike shorts or thin tights and a T-shirt) and if needed for the air temperature, more insulation like a sweater or fleece jacket over the wetsuit under your paddling jacket or dry-top, as well as warm socks, hat, and gloves or pogies.
For far more information on this subject, see Essentials of Sea Survival, Golden and Tipton, 2002, available at the Ballard branch library.