When a fresh water protozoan possessing a contractile vacuole, is placed in a glass containing marine water, the vacuole will-

A freshwater protozoan such as Amoeba or Paramecium lives in a hypotonic medium. Water continuously enters its body by osmosis.
The contractile vacuole collects this excess water and pumps it out at intervals (osmoregulation).
Marine water is hypertonic (or nearly isotonic) to the protoplasm. In it, water no longer enters the cell; instead the cell tends to lose water.
With no excess water to remove, the contractile vacuole is no longer needed. It stops functioning and disappears (marine protozoans generally lack contractile vacuoles).
So the vacuole will disappear.