I found these soldiers who were killed in action on-board Arkansas, July 15, 1862.
Later that day the Arkansas anchored below the batteries at Vicksburg.

They are not recorded in the listing of Confederates buried at Cedar Hill. I could hazard a guess that they were buried in a mass grave which markers later disappeared and were never found.

Private Charles Madden, Clinch's Battalion Artillery;
Killed on board C. S. Ram Arkansas, July 15, 1862.
Co. A, 22nd and 23rd La. Inf.
Private Henry Shields, Twenty-eighth Louisiana Volunteers, Company E;
Killed July 15, 1862 near Vicksburg
Private Antonio Florez, Twenty-eighth Louisiana Volunteers, Company G;
Killed July 15, 1862 in action on Ram Arkansas
Private Danl. Sullivan, Twenty-eighth Louisiana Volunteers, Company A;
Killed July 15, 1862 in action on Ram Arkansas
Private A. J. Blankenship, Missouri Volunteers, Company D;
Capt.Robert ”McDonald's" Co. MO 2nd Light Artillery later Co. D, 6th MO Inf.
Killed July 15, 1862 in action on Gun Boat Arkansas

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Setting sail with a crew of 100 sailors and 60 soldiers and commanded by Isaac Brown, the Arkansas steamed to Vicksburg, Virginia, where Farragut’s gunboats were rapidly dominating the river from New Orleans northward. At the mouth of the Yazoo River on July 15, 1862, the Arkansas engaged in a sharp exchange with the three Union ships sent to intercept the ironclad. After fighting through these ships, the Arkansas headed for the bulk of Farragut’s fleet. It then sailed through the flotilla, damaging 16 ships.
http://www.history.com/this-day-in-history/css-arkansas-attacks-union-ships

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The Arkansas runs through the entire fleet, trading fire with several Union ships, and anchors at last with no significant damage under the protection of Vicksburg’s big guns on the bluff.