Ocean waves are generated by wind, and large waves are the result of storms that have already occurred far out at sea. Large waves can travel long distances without losing energy from internal viscous effects (friction). However, waves do get smaller as they spread out laterally over the sea.
Wave generation and propagation can be predicted accurately with modern computer models that are often coupled with weather models. So, one can know days in advance what the wave conditions will be near the shore (at least for large waves from storms that have already occurred).
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