Answer: Contractile vacuole of amoeba will shrink/contract when the organism is placed in sea water.
Contractile vacuole is a subcellular structure ( mainly found in protists) that is involved in the regulation of water quantity inside a cell.
As we know that water always flow from a hypotonic ( where solute concentration is less) to a hypertonic ( where solute concentration is more) solution. Sea water is a hypertonic solution as the concentration of salt is very high. Therefore, in this case, water will flow from contractile vacuole to outside of the organism.
Thus, contractile vacuole of amoeba will contract as it will lose water when placed in sea water.