Platypuses are only related to seals and otters in that they are all semi-aquatic mammals. Even there, the similarity is flimsy, because seals and otters are placental mammals, whereas platypuses are monotremes, or egg-laying mammals, one of only two types of such animals in the world.
They are from the same super-family of Arctoidea, which in turn splits into the super-families of Pinnipedia (Seals, sea lions) and Musteloidea (otters, ferrets, weasels, etc)
If you mean what do otters eat, then they eat fish, and shellfish. Also, Giant river otters will occasionally attack a caiman wandering into their territory.
The Kwakiutl ate many aquatic type foods such as sea otters, fish, whales and seals. They also would hunt deer, birds and small game and gather roots and berries.