Sea otters usually prey on hard-shelled food. Clams, abalones, mussels and crabs. If they're too hard to crack with their teeth, they'll smash them open with, or against, a rock.
This is what this sea otter (Elfin) is doing when he bashes the ice against the rock. He's after the hidden gems of food frozen within the ice. Vancouver Aquarium's marine mammal trainers freeze seafood in chunks of ice and give them to the sea otters to play with, so they have a variety of ways to get their food.