Yes - when the Civil War began, the U.S. Navy remained in control of the Union (the United States), while the secessionist States (Confederate States of America), established their own Navy. Naturally, there were vessels and men who were a part of the United States before the split, which became a part of the CSA Navy, but the U.S. Navy officially was the Navy of the Union.
A regular US Navy Admiral, who fought for the Union.
Siezing control of the Mississippi River was an important goal of the Union Navy because it was the major means of transportation and logistical communications in the U.S. interior.
To have control of the boats and trade.
Navy Federal
At the beginning of the US Civil War, the US Navy had 90 ships. This does not represent the actual navy in that only 42 were commissioned vessels. It was also the wrong navy for the war the Union had to fight. The navy lacked the equipment and ships for use in coastal and inland waterways that dominated its needed deployment for the war. Any early Union blockade efforts were hampered by this. In addition, the Union lacked ironclad warships.
Gaining control of the sea surrounding the Guadalcanal area. Translation: Fighting the Japanese navy. Note: The US Marine Corps are part of the US Navy.
When the US Civil War began, the Union navy had less than 100 battle ready ships. In order to effectively blockade Confederate ports warship production went into overdrive. At the beginning of 1865, the Union navy had expanded to 670 warships.
No, they were isolated by the US Navy blockade.
The Union navy had naval superiority during the Civil War.
Riverine warfare; control of the rivers (like the Mississippi River).
By the word "posed," I assume you are asking about the kind of threat it posed towards the US Navy. "The C.S. Navy could never achieve equality with the Union Navy and used technological innovation, such as ironclads, submarines, torpedo boats, and naval mines (then known as torpedoes) to gain advantage over the Union Navy. In February 1861, the Confederate Navy had thirty ships, only fourteen of which were seaworthy, while the Union Navy had ninety vessels. The C.S. Navy eventually grew to 101 ships to meet the rise in naval conflicts and enemy threats."
US Navy warships. During the US Civil War, the US was called: Federal forces; Union forces; Yankees; Yanks; Blue Coats; Northerners; etc.