Answer:
The only predator that the small species of dolphins or calves have in the ocean are the large species of sharks.
Besides, there are no constant threats in the sea.
Sometimes, killer whales attack or eat dolphins, but this is not a standard behavior. In fact, killer whales belong to the family of oceanic dolphins too.
However, the most dangerous predators that dolphins have today are humans, and although we are not in the sea, we kill more dolphins (and sharks) than any other animal in the ocean.