Bottlenose dolphins, like all marine mammals, have a physiological adaptation to diving. The adaptation allows for the dolphins to dive longer by conserving oxygen. The heart rate is slower causing less blood to flow in turn conserving oxygen.
Morphological
The forelimbs of sea turtles are used as giant paddles when they swim. This is an easy adaptation for them to be very strong swimmers.
Behavioral
Many whales eat by swimming through a large group of fish/plankton/krill with their mouths open. This gets them the most food possible.