The people of the Marshall Islands used a number of techniques to help them navigate between their 29 atolls and 5 islands. Such as following the seasonal navigation of birds, celestial navigation and noting the interferences in air and sea patterns caused by atolls. They might also have taken frigate birds with them. These birds will not land on the water as their feathers will become waterlogged making it impossible to fly. When the navigators thought they were close to land they could release a bird, which would either fly towards land, or return to the canoe if no land was in sight.
Doctor David Lewis’ ‘We, the Navigators: Ancient Art of Landfinding in the Pacific’ is the classic study of Pacific navigation techniques.
Probably most of the travel in the Marshall Islands took place between the islands in individual atolls. Bikini Atoll has 23 islands and the central lagoon is 229 square miles. Enewetok has 40 islands and a central lagoon 50 miles in circumference. Navigating even within a lagoon would not have been trivial.
Navigating between atolls would in most cases have been both difficult and dangerous. The area bounded by Bikini Utrik, Majuro and Ebon is 165,000 square miles. That is a lot of sea to get lost in. Travelling from Bikini to Enewetok involves a journey of 190 miles and if a navigator missed Enewetok it was a long way to the next land.
Navigators were important people who guarded their knowledge. They created stick charts to help them remember the direction of other atolls and the currents and swells that would affect their navigation. The charts consisted of sticks tied to create a frame and shells fastened to the frame to indicate atolls.
Some stick charts just showed [usually inaccurately] the direction and distance of some other atolls, like this 1884 specimen one from a Swedish Museum.
And this one from the Harvard University’s Peabody Museum.
Others also included sticks to indicate the flow of currents and swells.
Each navigator prepared his own stick chart and probably jealously guarded it from other navigators, just as sailing masters guarded the charts and notes in the early days of Western exploration.
During the Cold War the US used the Marshall Islands for testing nuclear weapons. The islands were designated as the Pacific Proving Grounds. See this post on the testing that took place there. There is also a stick chart that was made to illustrate the geography and identify the main atolls used for testing.