As quoted http://www.awakin.org/read/view.php?tid=189 in  Mother Teresa's Reaching Out In Love - Stories told by Mother Teresa http://books.google.de/books?hl=de&id=tdyw409qGgQC&q=ocean#search_anchor, Compiled and Edited by Edward Le Joly and Jaya Chaliha, Barnes & Noble, 2002, p. 122 
2000s
                                    
“What we are trying to do may be just a drop in the ocean, but the ocean would be less because of that missing drop.”
Source: Three Cups of Tea: One Man's Mission to Promote Peace ... One School at a Time
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Greg Mortenson 7
American mountaineer and humanitarian 1957Related quotes
“All know that the drop merges into the ocean, but few know that the ocean merges into the drop.”
“What we know is a drop, what we don't know is an ocean.”
                                        
                                        "Communism and New Economic Policy",(April 1921) 
1920s
                                    
                                        
                                        Gandhi: His Life and Message for the World (1954), by Louis Fischer, p. 177 
 Mahatma Gandhi to Rajkumari Amrit Kaur, August 29, 1947 https://www.gandhiheritageportal.org/ghp_booksection_detail/Ny0yMzUtMg==#page/258/mode/2up. In Letters to Rajkumari Amrit Kaur. 1st edition (April, 1961), p. 246 
Posthumous publications (1950s and later)
                                    
“Jeff Buckley was a pure drop in an ocean of noise.”
Bono of U2 from Mojo Magazine, August 1997
Counterterrorism and Cybersecurity: Total Information Awareness (2nd Edition), 2015