Great White Sharks
Killer Whales(mainly in Alaska)
Humans both intentionally and unintentionally
Steller Sea Lions
Kelp provide shelter and are the producers in the their niche areas. Sea Otters are the keystone species in this habitat because of their relationship to the kelp and sea urchins. Sea urchins are the kelp forests biggest predators and will mow down kelp forests if left unchecked. Sea Otters are the ones who keep these devilish Sea Urchins from destroying the underwater forests. They feed on Sea Urchins as an important part of their diet. If Sea Otter counts drop too low, like during the 19th century, Sea Urchin populations will skyrocket and kelp forests will begin to shrink. With Sea Otters nearly driven to extinction, federal government took action to ban poaching of these animals. They succeeded and saved Sea Otters and kelp forests.
To help out Sea Otters visit http://www.defenders.org/sea-otter/basic-facts. They have a program called "Adopt a Sea Otter" along with some quick facts about them, too!