As a general rule—particularly in off-season kayaking when the air may still be warm but the water is still very cold—dress for the water temperature. You must assume you’ll go over and immerse yourself, and dress accordingly.
So, what’s cold? If you’re cold sensitive, then it’s quite reasonable to feel affects of the cold when the water temperatures are only 77 degrees F (25 degrees C) but on average it takes temperatures below 60 degrees F (15 degrees C) to become life-threatening. That’s not to say you can swim naked for hours on end in temperatures above 77 degrees F (25 degrees C) without being affected by the cold or die instantaneously of hypothermia the moment you plunge into water below 60 degrees F (15 degrees C). The severity of the situation directly after immersion in cold water has more to do with your “shock” value then hypothermia.
This is a key point made by the world’s leading authority on cold water immersion, Dr. Gordon Giesbrecht , aka “Professor Popsicle” …., director of the University of Manitoba’s Laboratory for Exercise and Environmental Medicine. He’s the “King of the Big Chill” and takes his job very seriously. “Professor Popsicle” has, for the goodness of science, lowered his body temperature below 95 degrees (the inception of hypothermia) over three-dozen times.
Giesbrecht’s studies show that the idea of immediately getting hypothermic the moment you take the plunge in cold water is false. The initial danger here is that instant gasp of breath one takes, which will end up drowning you if you submerge, inhale and panic. If you don’t panic, control your breathing, begin to slowly tread water or keep a hold of your boat, the “gasping” will subside within a minute or two. You will definitely go numb but the pain of the cold will lessen and you’ll have about ten minutes before muscles react and the real cold sets in. You have that time to get back in your boat or head for shore. After that point you’re mind may not be rational but your body has another hour before its reaches the critical point of being hypothermic.
Whether the danger lies in initially gasping for air or becoming hypothermic later on, it still means that if you dress proper for the weather then the hazards decrease dramatically. As a general rule, specific immersion wear is a must when water temperature and air temperature combined (in Fahrenheit) add up less than 100 degrees (37 Celsius). This is not to say, of course, that things like your experience, distance you are from shore, roughness of the conditions or if you’re traveling solo should not be included into the equation.