Sea turtles are strong swimmers. The cruising speed for green sea turtles is about 1.5 to 2.3 kph (0.9-1.4 mph). Leatherbacks have been recorded at speeds of 1.5 to 9.3 kph (0.9-5.8 mph).

Forelimbs are modified into long, paddle-like flippers for swimming.

Neck and limbs are nonretractile. The shell adaptations necessary for retractile limbs would impede rapid swimming.

Sea turtles are excellent divers. Leatherbacks routinely dive more than 305 m (1,000 ft.). They may reach depths of more than 1,190 m (3,900 ft.) seeking jellyfish for prey.

Since they are cold-blooded, sea turtles have a slow metabolic rate. This slowed metabolism allows them to stay submerged for long periods of time.

During long dives, blood is shunted away from tissues tolerant of low oxygen levels toward the heart, brain, and central nervous system.

Leatherbacks have high concentrations of red blood cells; therefore, their blood retains more oxygen. The muscle of leatherbacks has a high content of the oxygen-binding protein myoglobin. Myoglobin transports and stores oxygen in muscle tissue.

In studies conducted on green sea turtles, lung capacity exchange in one breath exceeded 50%.

Sea turtles can live in seawater with no need for a freshwater source. They obtain sufficient water from their diet and from metabolizing seawater.

Like other marine reptiles and seabirds, sea turtles have a salt gland to rid their bodies of excess salt. This gland empties into the sea turtles' eyes. The secretion of salt and fluid makes them look as if they are "crying" when they come ashore. These "tears" also help keep the eyes free of sand while females dig their nests.

For the most part, the only time sea turtles leave the sea is when females haul out to lay eggs. On some uninhabited or sparsely-inhabited beaches, turtles have been observed basking on land.

Many adaptations that make sea turtles successful in the sea make them slow and vulnerable on land.