Harbor seals swim with all four flippers: they move their hind flippers from side to side to propel themselves forward, and use their foreflippers to help them steer.

Harbor seals can swim forward and upside-down. They rarely swim backward.

Harbor seals can swim up to 19 kph (12 mph), but they generally cruise at slower speeds.

Harbor seals can dive to depths exceeding 200 m (656 ft.). They don't routinely dive this deep, however, since most of their food is found in shallow waters.

Adult harbor seals can stay submerged for up to 30 minutes, but dives usually last only about three minutes. A two-day-old harbor seal pup can stay submerged for up to two minutes.

All marine mammals have special physiological adaptations for diving. These adaptations enable a harbor seal to conserve oxygen while it is under water.

Before a deep dive, a harbor seal exhales to reduce the amount of air in its lungs. Oxygen is stored in the blood and muscle tissues, rather than in the lungs.

Like most other marine mammals, a harbor seal's typical respiration cycle is a short exhalation, a short inhalation, and a longer breath-holding (apnea) period.

Harbor seals sleep on land or in the water. In the water they sleep at the surface and often assume a posture known as bottling - their entire bodies remain submerged with just their heads exposed. This enables them to breathe when necessary.

A harbor seal's core temperature is about 37.8ºC (100ºF). There is a heat gradient throughout the blubber from the body core to the skin. The skin remains about one degree Celsius warmer than surrounding water.

Harbor seals have a metabolic rate somewhat higher than land mammals of the same size. This helps them generate body heat for warmth.

A thick layer of blubber insulates the harbor seal, reducing heat loss. The blubber of a northern Pacific harbor seal during winter may account for 27% to 30% of its total body mass. Blubber also streamlines the body and functions as an energy reserve from which the harbor seal can draw energy during periods of fasting. A harbor seal's hair provides no insulation.

In cold water, blood is shunted inward as blood vessels in the skin constrict, reducing heat loss to the environment.

When hauled out on land, blood vessels in the skin dilate, allowing heat to be released to the environment.