How do oceans regulate the climate?

"Oceans cover about 70 percent of Earth's surface. Their large mass and thermal properties, enable them to store vast quantities of heat. Oceans buffer and regulate temperature ? energy absorbed or lost by the oceans results in a smaller surface temperature change than would occur over land. The atmosphere and ocean constantly exchange energy and matter. For example, water evaporates from the oceans into the atmosphere. This moisture then falls back to the Earth as precipitation ? rain, snow, sleet, and even the morning dew on the grass."

By absorbing and releasing heat.  The oceans can store an enormous amount of heat.  The warm water of the Gulf Stream causes Europe to have a much warmer climate than it would otherwise.  (The warm water also causes hurricanes.)