Margot Lee Shetterly's book Hidden Figures is the fascinating story of black women who worked as "human computers" for NASA in the 1960s. The space program needed hundreds of mathematicians to perform the huge number of calculations the engineers would use to do their work.
NASA decided early on that it made sense to have space capsules come down (or "splashdown") into the ocean. This method was considered to be much safer than coming down on land. Ocean landings were also more convenient because space facilities like the Kennedy Space Center at Cape Canaveral, Florida, are located near the water.
Between 1965 and 1966, the Gemini program tested NASA's space capsule. The capsules were launched on top of a rocket before detaching and orbiting the Earth. The capsules used large parachutes to slow their descent before splashing down into the ocean. To increase the buoyancy of the capsules, flotation collars were added to them.
In the era covered by Shetterly's book, the Gemini 11 capsule landed in the waters of the Caribbean on September 15, 1966. The USS Guam was on station to recover the capsule and the astronauts. They were recovered successfully, but the effort was complicated a bit because the capsule missed the landing zone by about three miles.