How did the US coastline provide an advantage for the South during the Civil War?

It made it difficult for the Union to create blockades.

Southerners enjoyed the initial advantage of morale: The South was fighting to maintain its way of life, whereas the North was fighting to maintain a union. Slavery did not become a moral cause of the Union effort until Lincoln announced the Emancipation Proclamation in 1863.

Explanation:

none needed!