A healthy 21-year-old woman develops a profuse, watery diarrhea 1 day after a meal of raw oysters. On physical examination, her temperature is 37.5deg C. A stool sample is negative for occult blood. There is no abdominal distention or tenderness, and bowel sounds are present. Diarrhea subsides over the next 3 days. Which of the following organisms is most likely to produce these findings?