Water always moves down the slope, i.e., from higher elevation to lower elevation. And sea is always at a lower elevation. So water flow down to the sea.
We know that water(liquids) always flow down from a higher to a lower surface and since sea is lower than hills the above mentioned phenomenon occurs .This happens because of 2 facts:
1.Force of Gravity.
2.Liquids possessing translatory motion slip and slide against each other to flow down from a higher to a lower level.
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