Contractile vacuoles are used to pump water out of the amoeba. In a freshwater environment, the inside of the amoeba is hypotonic for water to the surrounding water. An osmotic gradient is therefore present from the area of high osmotic pressure (the surroundings) to the area of low osmotic pressure (in the amoeba) and so H2O enters the cytoplasm via the semi-permeable cell membrane. The marine one doesn't need this because it is isotonic to its surroundings for water due to the saline content of the surroundings.

I know. *grumble* You don't care. :P