From 1858-1859, the USS Arctic was taken out of service and converted back into a lightship at Norfolk and turned over to Lightship Services. Its engine and boiler were placed in storage. After Norfolk was seized by Confederates it is possible that they were sent to Wilmington, N.C. for use on an ironclad.
Now as the Lightship Arctic, it was placed in service off Wilmington where it too was seized by Confederates in 1861 and converted into a floating battery/guard ship and became the CSS Arctic. It is possible that the original sheet iron on its hull was still in place which might be why some classified it as an ironclad floating battery. However, sheet iron wouldn't help much in stopping a shell.
It is also possible that more sheet iron was placed on the hull.
It likely also had three cannons arranged along center pivots in open mounts.