The U.S.S. Water Witch had been used as a mail (or dispatch) ship until shortly before the Battle of the Head of Passes and its armament seems to have been rather light during that time.
In A.T. Mahan's "The Navy in the Civil War, Volume III, The Gulf and Inland Waters," he writes of the Water Witch at Head of the Passes the following:
"It is a relief to say that the Water Witch, a small vessel of under four hundred tons, with three light guns, commanded by Lieutenant Francis Winslow, held her ground, steaming up beyond the fire-rafts until daylight showed the larger vessels in retreat." [page 7]
So what were these three light guns?
Lieutenant Francis Winslow, commanding the Witch, reported receiving "an additional howitzer" on September 19, 1861 prior to a reconnaissance to the Head of Passes on the following day where it engaged the C.S.S. Ivy. Winslow reported only using a rifled howitzer in the fight.
"Twenty-three projectiles were thrown from the rifled howitzer, some of them falling quite close to the steamer…" [Official Records of the Union and Confederate Navies, Series I, Volume 16, page 683].
At the actual Battle of Head of the Passes on October 12, Winslow included in his report:
"…apprehending a design on the part of the enemy to run the bark out to sea by the Pass a l'Outre (which the light howitzer battery of the Water Witch would have proved inadequate to prevent), she was now steered after the Richmond at full speed…" [O.R. N. I,16, page 715]
Like the Witch's earlier brush with the Ivy, Winslow again only reports firing his rifled howitzer on the 12th. And again at the Ivy.
"Signal to 'engage the enemy,' being also made to the Water Witch, our efforts to relieve the Vincennes were discontinued, and our rifled 12-pounder howitzer (the only gun of adequate range on board) was brought to bear on the Ivy." [O.R. N. I,16, page 716]
So it seems that out of the three "light guns" on board, the rifled 12-pounder howitzer was the only one used or even worthy of mention in the reports.
In the "Statistical Data of U.S. Ships" ( O.R. N. Series II, Volume 1, page 237) The Witch's armament for June 16, 1863 [there is nothing listed earlier] was "1 30-pdr. Parrott rifle, 1 12-pdr. rifle, 2 heavy 12-[pdr]." With the exception of the one rifled 12-pounder, this is the same as listed by Canney. I have yet to see a source for Silverstone's 32-pounder.
Based on the above, for your Order of Battle for Head of the Passes, I would give the Water Witch the rifled 12-pounder the record says was there, and then deduce that the other two light howitzers were smoothbore 12, or maybe even 6 pounders.
