Configuration of Oceans and Continents
If the Earth's surface were 100% ocean, weather would appear in belts and zones (reflecting the structure of the atmospheric cells), and the planet might look like some blue-and-white mini-Jupiter. But the Earth has continents, and the flow of ocean water is restricted by their presence.
Like air, moving water is also subject to the Coriolis Effect; thus, water confined to an ocean basin tends to circulate in a huge gyre, clockwise in the northern hemisphere and counterclockwise in the southern hemisphere.
Warm-water currents from the tropics, like the Gulf Stream in the NW Atlantic and Kuroshio in the NW Pacific, carry warm water to high latitudes, where they cause precipitation. Warm water at high latitudes cools, and is forced towards the equator. These cold-water currents moving towards the equator--like the California Current in the NE Pacific, and the Peru or Humboldt Current in the SE Pacific--chill the air and cause dry, "Mediterranean" climates on west coasts at mid-latitudes. Westerly winds, blowing over western coasts, can carry the moderating effect of the ocean inland (unless blocked by mountains), but also carry the more extreme effect of the continental interior out to the eastern coasts.