Take note of the scene where Superman is stabbed and falls a long way into the water. (1:57:39) It is clearly the ocean he falls into, and not a body of water on the crystal island. Later on, when he is rescued by Richard and Lois (and the kid), the seaplane goes over a really tall waterfall. There couldn't be two successive waterfalls. (Nevertheless, the dramatic effect is worth this slipup.)