Suddenly one dived down, and came up with a pawful of mussels. It couldn't get into all of them, so it dived again, bringing up a stone which it carried on its stomach. In a flurry of water it began to break the shellfish open on the stone, quickly (and noisily) slurping at the opened flesh. We stood there for a while, just watching it feed.
The otter would dive for more shells, holding the stone close to its chest as it dived, ready to use for the next batch of mussels. Down, up, crunch, bang, slurp. Occasionally it would tangle in some floating weed, ignoring the encumbrance as it concentrated on feeding.
Sometimes it's like living in a wildlife documentary.