She was inspected and accepted by Captain Lawrence Rousseau, CSN, on April 22, 1861, acquired by the Confederacy in June, cut down and strengthened by hog frames and armed as a gunboat — all, apparently, in the Lake Pontchartrain area.

Her upper deck was plated at this time with 3⁄8-inch (9.5 mm) iron, partially protecting her boilers, of the low pressure type preferred for fuel economy and greater safety in battle.

The Mobile Evening News editorialized early in December on the startling change "from her former gay, first-class hotel appearance, having been relieved of her upper works and painted as black as the inside of her smokestack.

Fortunately for her, the coast was clear of Union ships and batteries, for Florida fouled the area's main military telegraph line with her anchor, and had no sooner repaired the damage than she went aground for 36 hours.

The havoc caused by one well-placed shot with her rifled pivot gun is described by Commander Melancton Smith, USN, commanding Massachusetts; It entered the starboard side abaft the engine five feet above the water line, cutting entirely through 18 planks of the main deck, carried away the table, sofas, eight sections of iron steam pipe, and exploded in the stateroom on the port side, stripping the bulkheads of four rooms, and setting fire to the vessel ... 12 pieces of the fragments have been collected and weigh 58 pounds.The first sortie by Florida caused consternation.

The caliber and long range of the rifled cannon from which the shell that exploded in the Massachusetts was fired established the ability of these fast steam gunboats to keep out of the range of all broadside guns, and enables them to disregard the armament or magnitude of all ships thus armed, or indeed any number of them, when sheltered by shoal water.Protecting CSS Pamilico, in contrasting white dress and laden with some 400 troops, "the black rebel steamer" Florida on December 4 had a brush with USS Montgomery in Horn Island Pass that caused jubilation in the Southern press.

Commodore McKean promptly sent Lieutenant James Edward Jouett to relieve him and forwarded Shaw's action report to United States Secretary of the Navy Gideon Welles, noting, "It needs no comment."

Crowed Richmond Dispatch on December 14, quoting Mobile Evening News, "The Florida fought at great disadvantage in one respect, owing to her steering apparatus being out of order, but showed a decided superiority in the effectiveness of her armament.