Orcas are a natural predator of everything that his the ocean. Fun fact, orcas have been known to toy with seals by catapulting them with their tails. I believe I remember seeing at least one baby seal got seventy feet in the air before returning to the sea (and its inevitable death).
All cases known to have happened are in Alaska, where moose were swimming across straits and between islands. Orcas are opportunistic hunters and nearly anything swimming in water deep enough for them to swim in has a chance of being eaten. Most of them keep their distance from humans but if you were swimming in their territory away from civilization and boat traffic you might be stalked and hunted.