Possible Clues:

• It's used as a bulwark against flood
• Unexpectedly attack son with net
• Strong-arm
• Filled sack used as ballast
• Son with container in flood defence
• Some ballast initially, a good defence against flooding
• Filled sack for use against floods, etc
• Possible ballast to drop, with balloon initially secured?
• Polish to get the sack for blockage
• Emergency levee component