A keel cooler assembly comprising a liquid coolant tube including a plurality of turbulence enhancers for improving the heat transfer of the liquid coolant without substantially increasing pressure drop of the liquid coolant. In one embodiment, the turbulence enhancers generate turbulent wakes in the liquid coolant for disrupting laminar boundary layers for improving heat transfer. In another embodiment, the turbulence enhancers generate and propagate turbulent vortexes in the liquid coolant to enhance mixing of the bulk liquid coolant for improving heat transfer. In other embodiments, turbulators, including inserts or impediments, are provided having various configurations and being arranged in predetermined patterns for enhancing turbulence of the liquid coolant for improving keel cooler heat transfer efficiency without substantially increasing pressure drop.