The ocean's surface currents move warm and cold water around the world, affecting air temperature above the currents and moisture content. Surface currents have the biggest affect on the climate of coastal regions.
Because oceans cover more than 70 percent of the planet, they also absorb the majority of the heat energy on Earth. Driven by winds, gravity and the Earth's rotation, surface currents circulate warm water up and down toward the poles. The equator would be much hotter and the poles significantly colder without this constant circulation of heat.
Warming from surface currents explains why coastal areas at similar latitudes can have widely different climates. Cold currents tend to circulate on eastern coastlines, while warm currents tend to circulate on western coastlines.