Hello Everyone,
I believe the first iron-hulled warship lost was the gunboat R.B. Forbes which was wrecked off Virginia on February 25, 1862. The first ironclad lost due to combat was the Manassas on April 24, 1862. Technically she was run aground and abandoned after being made 'hors d' combat' (an amusing contemporary phrase) but later slid off the bank and sank. I would have to go with Andy with the Hatteras as the first iron-hulled warship sunk by combat on open waters. The first iron-hulled ironclad sunk due to combat would be the Tecumseh. Both the Monitor and Weehawken foundered but not related to battle.
All the best,
Bil