Simply put, practically any place that is a short distance from an ocean is seacoast terrain.

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In these rules, the distinction is important for determining weather conditions, the availability of animals (for food) and
the availability of plant life (for both food and medicinal purposes).
If the DM uses encounter tables such as those in the DMG || MMII for determining possible confrontations with wildlife or
other residents of the AREA, the seacoast AREA should be considered
as some other type of terrain that conforms to the classifications in the encounter tables.