Would definately agree with mj345 and HNTFSH on using collar pressure on a young dog in cold water.
Take a look back at Nicks' Chessie pics. I'm a poor man, but would bet my next paycheck that Thor hasn't seen any collar pressure in a long, (long), time. Based on my own huntin' experiences with dogs in cold arse water for me,.. it's really more about how a dog "feels" about retrieving in general, and the intimate relationship the dog has, (or doesn't have) with birds.
Have seen properly FF'd dogs scarf at cold water as if it wasn't written in the "contract". Particularly after having the luxury of nice, warm water to train in most of the time, and then a lapse in training as huntin' season nears. THEN the dog is suddenly asked to retrieve in water that it's just not acclimated to.
Best thing I could suggest is to keep a dog active in water from Sept. thru fall and into winter as water temps continue to drop.
Will never believe that FF' creates a lifelong passion for a dog to "like" cold water.