A seawall is a bulkhead at the edge of a body of water. It provides a straight edge where boats and ships can moor. It brings deep water closer in-shore and protects the land from erosion. It sharpens the transition between land and water, which in nature is usually fairly gradual.
One online Dictionary.com/seawall places the emphasis on erosion protection and indeed seawalls can be built for that express purpose. But the Alaskan Way seawall in Seattle was built as a way to permit ships close to the shore of Elliott Bay for easier loading and unloading. Of course the "working waterfront" has disappeared over the past 50 years almost entirely from the shoreline of Seattle's historic urban core; the Puget Sound fishing industry is virtually dead and international shipping has moved to areas with large expanses of land for marshaling of containers.
But the seawall remains and there is concern that it may need substantial repair.
I'll post a picture of part of the Alaskan Way seawall shortly.