The ocean currents that have a great effect on climate because of water are the "warm" ocean currents and the "cold" ocean currents.
Warm ocean currents are those that transport warm water from the tropics towards higher latitudes, while cold ocean currents transport cold water from high latitudes towards the equator. The warm ocean currents can help to moderate the climate of regions located at higher latitudes, making them warmer and wetter than they would otherwise be. The Gulf Stream, for example, is a warm ocean current that has a significant impact on the climate of the eastern United States and western Europe.
On the other hand, the cold ocean currents can have the opposite effect, making the climate of regions located at lower latitudes cooler and drier than they would be otherwise. The California Current, for example, is a cold ocean current that has a significant impact on the climate of the western United States.