Some individuals remain in the most southerly parts of the range throughout the year, though they are less obvious in winter, when they presumably stay in the water where the temperature is constant. Leptonychotes depends heavily on holes through the ice for access to the water and for breathing. Holes are made initially by cutting with the incisor teeth and then are enlarged and maintained by abrading the ice with the canines. Cutting does not begin in areas where the ice is over 10 cm thick, but once a hole is made, it may be maintained even if the ice increases to a thickness of 100-200 cm. Holes generally are made near natural cracks in the ice, and Leptonychotes, while underwater, evidently uses the radiating pattern of cracks as a guide to find its way to the holes.
Underwater vision is excellent and is thought to be the primary means of navigation.
Dives are deeper and more frequent in daylight. During the long period of winter darkness, dives tend to be shallow and probably are in areas with which the animals had become familiar previously when light was available. The underwater activity of the Weddell seal has been investigated by capturing some individuals, attaching an instrument known as the time-depth recorder, and then releasing them. Animals tend to make an initial series of shallow dives to familiarize themselves with an area and then begin to go farther and deeper. The entire series may go on for 8-9 hours without stopping.