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.