The United States Coast Guard is already a branch of the military, and already has responsibility for ensuring maritime safety of both ships and people in and around American waters, including ports. It even operated Navy ships during World War II, defending convoys and battling German submarines. So, what if the Coast Guard had gone on to take up the trade protection, ASW, and mine removal roles held by the Navy? This would probably take place sometime around the world wars, or possibly during the Cold War. Historically ASW and mine removal hasn't been a priority of the Navy (it's one of the areas the Navy is considered to be weak in), so might something like that have improved those capabilities?