But geeks don't mind getting geeky. The icy water will be more dense at 32 F than at, say, 70 F, so that cold water would impart more buoyancy. It's like being more able to float in salt water. But for fresh water, the density is greater at 32 F vs 70 F, but only by 1/4 of 1%, of no real consequence.
But for a gas inflating a vest, the gas law does make a difference in volume of the vest and thus of buoyant force provided. At low pressure, the volume is about proportional to absolute temperature. For temps in Fahrenheit, add 459.67 to get Rankine, and take the ratio of absolute temperatures. So the gas volume at 32 F is only 87% that at 70 F (winter vs summer temps, approx.). However, CO2 is a bit weird, so an equation of state is more appropriate. Here is a calculator for density of CO2:
http://www.energy.psu.edu/tools/CO2-EOS/, from which the density ratio, winter vs summer, is 0.927.
But with a CO2 cartridge, things get more geeky. Under pressure, there is liquid CO2 in the cartridge, with a gaseous CO2 vapor space. When released, the gas expands into the vest, but the pressure in the cartridge drops, causing vaporization of liquid CO2. The heat of vaporization comes largely from the CO2 itself, resulting in a large drop in temperature, and perhaps in some solid CO2 (dry ice) flakes. Eventually, the cold CO2 gas that has expanded into the vest will pick up heat from the cold water and expand more, but initially the inflation will be substantially less than at thermal equilibrium.