Honestly?

Motley and not all that objectively powerful. The neglect in the years before the war, focus on riverine and coastal operations, interest in hull numbers over quality, and neglect again after the war, meant that by the mid 1880's Brazil had a more powerful fleet than the United States.

While the USN had around 650 ships in service by 1865, many capable of ocean voyages, most were not purpose built warships. Many a blockade ship was simply a private vessel, be it cargo hauler or even yacht, with a gun or two mounted and a crew enlisted. Against even more lightly armed blockade runners that was sufficient.

The USN did also commission an emergency shipbuilding program as well, mostly to turnout light sloops of war that could provide some additional backbone. While the 40-ish ships in active service at the start of the war were joined by many of the 50 in various manner of storage. These provided much of the heavier hulls like the steam frigates Minnesota or screw sloops like the Hartford. Others however were still all sailing ships such as the Congress and Cumberland involved in the battle of Hampton Roads. While useful and still valued, the lack of alternative propulsion was increasingly a hindrance no matter how many guns the ship mounted.

War emergency ships also tended to be under armed in comparison to international peers. For instance the famous USN sloops the Housatonic(attacked by the Hunley) and Kearsarge(sank the CSS Alabama) mounted between 7-12 guns of various sizes and utilities. The similarly sized Camelion class sloop of 1860 for the Royal Navy then mounted 17 guns for comparison. And while the evolution of center mounted rotating guns mad this a non issue to a degree ships still retained sizeable broadside mountings which could and would do damage. This was compounded by the fact that the Dahlgren guns mounted by most USN ships was not necessarily able to keep pace with the armor penetration power of newer and more powerful British designs.

And while many of the coastal and riverine ironclads such as the famed Monitor did give good service, they were simply not up to the equivalent British and French designs.

In 1858 France laid down the seagoing ironclads, the wood hulled Gloire class of 3 ships. The first oceangoing ironclads these ships were no joke, that is until Britain answered with 2 truly monstrous ships. HMS Warrior(still around as a museum ship) and Black Prince were 10k tons of iron hull, heavy guns, and death for any other warship on earth at the time. Meanwhile compared to the two notable USN attempts at similar armored frigate designs, the Galena and New Ironsides, Warrior was nearly twice the size, mounted more than double the guns, and could go nearly twice as fast. HMS Warrior today

And by 1865 the RN and French Navy each had about a dozen seagoing ironclads of carious sizes and a combination of new builds and existing ships either cut down or otherwise modified and armored. If war had come between the US and one of those powers it is beyond a doubt in my mind that either fleet could have lifted the blockade on a Southern port of their choice or blockaded a Northern port of choice. While actually taking or closing completely is a whole other question, this was during the era that the RN still ruled supreme and had basically operational impunity.