Impede que a água entre no motor do barco. Encontrei páginas em português sempre usando o termo em inglês mesmo, sem tradução.

While the engine is stopped, the waterlock collects the water in the exhaust and forms an air lock . The water is contained so that it doesn't shoot back into the engine while sailing or at anchor. The 'lock' prevents salt laden air from flowing backward and forward through the engine thereby preventing valves, valve seats, etc. from corrosion. It also reduces exhaust noise.

http://www.yanmarhelp.com/i_exhaust.htm