It is highly unlikely that someone could see a fish swimming in their stomach. Our stomachs have been made to digest food using muscles and strong acids that could cause death to any live fish. In addition, the stomach and digestive tract aren’t sensitive to movements in the same way as our muscles or skin care, so the sensation of a swimming fish is not felt similarly. But, if a living fish is swallowed and is living when inserted into a stomach, it might be able to continue moving because of the powerful constriction of digestion. Of course, this can cause pain or discomfort, but it won’t be like the animal was swimming. In rare instances, when an enormous fish is stuck in the throat or esophagus, the patient feels an ache of something moving or getting stuck within their mouth. It could cause choking or other serious issues and require urgent medical treatment. While it is possible to ingest an entire fish in its natural state, however, it is not likely that anyone could see it swimming inside their stomachs because of how the digestive system works.