Otters are excellent swimmers. They swim with movements of the hind legs and tail. Freshwater otters "dog paddle" with all four feet when swimming slowly or floating. When swimming at a high speed, the entire body, including the tail, undulates up and down and the hind feet steer.

Researchers have observed giant otters swimming at speeds up to 14.4 kph (9 mph).

Sea otters spend a great deal of time floating on their backs at the surface. They move by paddling their hind limbs and sculling with their tails. For rapid swimming and for diving, they swim similarly to other otters, using up-and-down undulations of the body. Sea otters can reach speeds of 1.5 kph (0.9 mph) on the surface and 9 kph (5.6 mph) under water.

River otters can remain under water for 6 to 8 minutes.

Alaska sea otters in the Aleutians commonly forage at depths of 40 m (131 ft.) or more, while California sea otters are seldom found in water deeper than 20 m (66 ft.). Sea otters usually remain submerged for 52 to 90 seconds; the longest recorded dive was 4 minutes, 25 seconds. Such extended dives are probably a response to danger. The deepest confirmed dive for a sea otter was 97 m (318 ft.).

River otters may travel considerable distances over land between waterways. Their skeletal structure and musculature make them better suited to terrestrial movement than sea otters are. River otters can attain running speeds up to 47 kph (29 mph). They travel over snow and ice using a combination of running and sliding.

Sea otters are quite awkward on land, and spend the majority of their lives in the water.

Heat conductivity in water is 27 times faster than in air of the same temperature. Even in temperate water mammals lose body heat rapidly.

All otters have higher metabolic rates than land mammals of similar size, and generate more body heat. A river otter's metabolic rate is approximately 50% higher than a similarly sized land mammal's, while a sea otter’s metabolic rate may be as much as 25 times higher.

The higher metabolic rate necessitates a higher caloric intake.

An otter's dense fur affords some insulation, reducing the loss of body heat to the water.

For sea otters, maintaining a constant body temperature is particularly challenging. Alaska otters typically inhabit water that may be as much as 15.5 ºC (60 ºF) below their core temperature.