Factual error: During the launch sequence, a car that appears to be from the 1990's is parked in the background. (00:30:15)
Factual error: When astronauts are wearing their spacesuits before launch, we can see NASA's "snake logo" introduced in 1975, when the plot is happening in 1970. (00:28:29)
Factual error: On July 20, 1969, the moon phase was waxing crescent. In the movie, on the eve of the moon landing, the moon is shown in waxing gibbous phase when Lovell (Hanks) covers the moon with his thumb. (00:06:02)
Answer: Spacecraft re-enter Earth's atmosphere at extremely high velocity (thousands of miles per hour). Atmospheric friction slows the spacecraft descent somewhat; but, without parachutes, the Apollo spacecraft would still reach the surface traveling at hundreds of miles per hour. Landing in water at such high speed would be like hitting concrete, which would of course be instantly fatal. Hence the necessity of multiple parachutes. The Apollo program (and all early U.S. manned space programs) chose to land in the ocean for two reasons: 1) It was easier to track spacecraft re-entry from horizon-to-horizon at sea without visual and radar obstacles, and; 2) It was faster and easier to position several Navy vessels in the general splashdown location, then deploy helicopters to rapidly retrieve the astronauts and their spacecraft.
Charles Austin Miller