I saw on the recent A&E Biography episode about Samuel Clemens that a possible explaination of the name "Mark Twain" was that when he traveled out west, he frequented a particular saloon where upon ordering a drink he would hollar to the bartender, "mark twain!" The bartender was keeping a count of drinks ordered and "mark twain" was the riverboat enthusiast's way of saying "put it on my tab." I thought this was very interesting. Before that explanation I had always been taught that "Mark Twain" was the riverboat captain's call for, as you stated, safe water. I hope this is what you are looking for. -katie