A black dog is swimming with a ball in his mouth.
A black dog found a ball in the water and is bring it back to its owner.
A black dog found a ball in the water and is bring it back to its owner as the black dog was swimming with a ball in his mouth which must be because the dog found a ball in the water to be retrieved by the dog's owner.
Swimming implies in the water, and ball in his mouth implies bring it back to its owner
Swimming can only be done in the water