Just adding to @ABraun’s explanation: ships are moving, so highly accurate position at time of overpass doesn’t add much insight to a ship’s current position. In some areas (Canada’s Bay of Fundy) tides could have an impact. If there is a stationary target like an oil platform in your region you can use that to check your detections.
See: doi:10.3390/rs70709431 “Sentinel-1A Product Geolocation Accuracy: Commissioning Phase Results”