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
H<font size=-3><sub>2</sub></font>O 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.

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