Your head is the only visible indicator for Search And Rescue or approaching boats.

If no one knows you’re there, it could be a long wait in cold water. Even with a flight plan, the search won’t begin until 30 minutes after the flight plan expires—and then it is by radio and telephone. Search-and-rescue forces may not be activated until three hours after your estimated time of arrival, and the search area may be quite broad. Your odds improve if you could be seen from far away or above.