All mustelids have long sinuous bodies, which makes them ideally adapted to semi-aquatic life (minks are the only other semi-aquatic mustelids). Underwater, the otter holds its legs against the body, except for steering, and the hind end of the body is flexed in a series of vertical undulations.
River otters have webbing which extends for much of the length of each digit, though not to the very end. Giant otters and sea otters have even more prominent webs, while the asian short-clawed otter has no webbing - they hunt for shrimps in ditches and paddy fields so they don't need the swimming speed.
An otter's tail (or rudder, or stern) is stout at the base and tapers towards the tip where it flattens. This forms part of the propulsion unit when swimming fast under water.
River otter fur consists of two types of hair: stout guard hairs (up to 20mm long) which form a waterproof outer covering, and underfur which is dense and fine, equivalent to an otter's thermal underwear!
The fur must be kept in good condition by grooming. Sea water reduces the waterproofing and insulating qualities of otter fur when salt crystals form in the fur. This is why freshwater pools are important to river otters living on the coast. After swimming, they wash the salts off in the pools and then squirm on the ground to rub dry against vegetation.
Sea otters have the densest fur of all mammals, and because they live 100% in the sea their fur does not lose its insulation due to salt water. However, they do have to spend a lot of time grooming!
High metabolic rate
River otter metabolism is 20% higher than other similarly-sized animals to compensate for losing heat in the water. Despite this, they get chilled if they stay in the water too long. Coastal otters will only fish for 15 minutes at a time.
The otter's very large lungs allow it to remain underwater for up to 4 minutes if need be.