Dolphins have to be conscious to breath.
This has been determined by doing EEG studies on dolphins. Dolphins sleep about 8 hours day in this fashion. The short answer to this is that we do not know . There is no reliable method to measure intelligence in humans across cultures, so it is not surprising that comparing humans, dolphins, apes, dogs, etc. is impossible. The compostion of the diet depends very much on what is available in the area they live in and also on the season.