Though it is done, and I have done it, float flying in air temperatures lower than freezing is not a great idea. A splash of water during takeoff or landing will quickly freeze in undesired places. And, if you end up in the water, a life jacket will only do you good for minutes. An immersion suit is appropriate in such conditions. If your float plane is floating at the time of a big snowfall, the weight of the snow can sink it, another good reason to have it out by then. Most float operators I know try to have their float flying done and planes out by freeze up.