Deliberate mistake: Near the end of the movie, there is a shot of the "Iwo Jima" with the two recovery helicopters taking off. For a moment, you can see the number 11 on the inboard side of the ship's island, which identifies it as the USS New Orleans (LPH-11), a sister of the Iwo Jima. Admittedly, it was the best the producers could do, since by the time the movie was made, the Iwo Jima (LPH-2) had been decommissioned.
Deliberate mistake: During the scene where the astronauts are building the makeshift CO2 scrubbers, the camera zooms out of the lunar module through the window, and we can see that the interior of the lunar module is oriented incorrectly compared to the exterior. The tunnel leading to the command module is on the top in the interior shot, but on the left in the exterior shot.
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