It was after the famous Challenger Expedition’s observations in physical and biological marine sciences that other nations took interest in oceanographic research and started to organize their own expeditions. Since 1885, Prince Albert I of Monaco (1848-1922) conducted oceanographic campaigns (1885-1915).The accounts of the scientific results were published in the “Résultats des campagnes scientifiques” and in several articles and books, containing drawings, engravings and photographs as illustrations. He had the collaboration of some of the best scientists in marine biology and physics oceanography. Oceanographic maps and charts were recorded. Many techniques and instruments were also devised for this oceanographic work, some of the scientific instruments used methods of photographic self-recording. The intense scientific activity of Prince Albert, certainly contributed to stimulate the interest of King Carlos I of Portugal (1863-1908) for the study of the oceans, its cartography and marine species. Although some oceanographic work had already been done off the coast of Portugal by the Challenger in 1873 and by the Prince of Monaco in 1894 at the islands of Azores, King Carlos felt the need for a methodical study of the distribution and habitat of fishes, research on new marine species as well as the construction of bathymetric charts. He engaged the collaboration of the naturalist Albert Girard in all his expeditions (1896 – 1906).The general results of his campaigns were published in two works. Two other scientific publications were also published, one on Algarve tuna and another on sharks. The two monarchs exchanged information concerning the instruments and methods used in their oceanographic work which is well shown in an abundant correspondence between them during the years 1894 -1907. King Carlos took photomicrographs of marine plankton and documented photographically biological species as well as the activity on board the ships. The collections, instruments and biological species, the result of their oceanographic expeditions, were presented in several national and international exhibitions, thus contributing to the public awareness of marine life and conservation problems. In this paper we will discuss how photography was used in King Carlos oceanographic expeditions, as a document-object and as an aid to his research.