Seals are the primary predators of the penguin in the water. Penguins fear seals so much that they will jump out of the water if they see a dark spot. One seal was found to have 18 penguins in their digestive tract. Orcas are also penguin predators. They, however, are not as great a threat to penguins because they eat a variety of food.
Mankind is most destructive to penguins especially the humboldt pengins.
The humbolt penguins eat anchovies, krill, squid and fish. Heavy anchovy harvesting by humans has seriously affected their main food supply. They're also been exploited by having their rookeries invaded because their guano is valued as a commericial fertilizer. Their eggs have been taken for food and a demand for their oil, meat and skin has resulted in their death. At present though humans are trying to reverse this process.