The oceans play a role in the earth's weather systems through interactions with the atmosphere and the sun. First, the ocean, especially near the equator, which is the part of the earth closest to the sun, absorbs most of the sunlight. As water heats up, it evaporates into water vapor into the atmosphere. Again, this occurs the most in the tropics, near the equator. The evaporated water vapor condenses back into water droplets and eventually precipitates as rain. The process of condensation releases heat, which warms the air and draws more warm, wet air in. This drawing of air, due to the difference in air pressure, causes winds, which can carry weather systems away from where the water originally evaporated.
Water and land receiving the same amount of sunlight heat up differently. Water is better at storing heat, meaning a lot of sunlight is required to cause a change in water temperature. This is what drives monsoons. In the summer, the land is much warmer than water, causing moist air to be drawn inland and heavy rains to occur.