Most ocean waves are created by the wind blowing over the sea surface. The wind’s frictional drag gathers up the water and turns it into waves that get taller as they travel towards the shore. As they grow, the waves become more unstable, with the force of gravity tugging at their tallest, weakest points. This causes the crests of the waves to break apart into a mass of droplets and bubbles, which scatter the surrounding light in every direction, creating the familiar white crest of a breaking wave.
[Source: BBC Science Focus]