The way that sea turtles swim is unique for turtles. They are the only turtles whose front limbs are stronger than their back limbs. They use the powerful front flippers to propel themselves through the water. Contrast this to freshwater turtles, whose hind feet propel them through the water with alternating kicks. By sweeping their flippers up and down at the same time, sea turtles fly through the water. Like a bird's wing, each front flipper generates lots of thrust on the down-stroke and a lesser amount on the upstroke. The whole flipper cycle takes only about a second. The result is a powerful swimmer who can cover a lot of distance. Sea turtles regularly travel hundreds or thousands of kilometers from feeding areas to nesting beaches.

Turtles reproduce sexually, and sea turtles must do so in the ocean. The male courts a female by swimming around her, nuzzling and biting her shoulders, neck, or even flippers. During mating he hangs onto her top shell (carapace). Mating can go on for several hours, with male and female intermittently clambering to the surface for air. Afterward, a female heads for a nesting beach. With a body designed for water, not land, she spends a lot of energy hauling herself up to where she will dig a hole and deposit her eggs. Where she puts her nest is important not only for safety of the eggs, but also the sex of her hatchlings. Eggs hatched at lower temperatures will produce females, while those at higher temperatures will produce males. This temperature-dependent sex determination has been found in some other types of turtles, but with variations on what range of temperatures produce each sex.