Why the 1890s United States needed a fleet as well as forts

Could the United States defend its 1890s coastline with forts or coastal-defence ships alone?

No; it needed an oceanic fleet to control the main approaches, backed by forts and local ships as a final defensive layer.

The length of the American coastline made control of its approaches a fleet task. Sailing ships, and some steamships using tides and currents, converged on particular routes and trade points. An ocean-going force could hold those positions and intercept an enemy before it reached the coast. Forts and coastal-defence ships would then provide a second layer if an enemy fleet broke through.