Abstract: Apparatus for killing marine life, such as zebra mussels, in the cooling system (14) of a boat having a water-cooled internal combustion engine (11) includes a flexible bag-like enclosure (20) which is adapted to be mounted on the boat so as to provide a closed volume of water communicating with the cooling system inlet (18) and cooling system outlet (19). When the engine is allowed to idle in neutral, cooling water is drawn from the enclosure and is circulated through the cooling system in heat exchange relation to the engine, and is discharged back into the enclosure. The temperature of water in the enclosure and cooling system will therefor increase. After the engine has been operated for a period of time, the temperature of water in the enclosure and the cooling system will increase to a level necessary to kill marine life in the cooling system.