Dolphins sleep by either swimming slowly and surfacing for breath or by resting at the surface of the water with their blowhole exposed. They can also rest in the bottom of the water as long as it is shallow. You can find more information here: http://www.dolphinear.com/data/dolphins.htm
It is called logging, when the dolphin is completely still and floating it resembles a log. They do this while floating and to avoid drowning they retain control of their blowholes. Most Dolphins sleep about 33% of the day, they are most active at night for feeding.