Correction: The weapon is a light 12-pounder Dahlgren boat howitzer. I first thought it was the rifle, but those were all on the heavy boat howitzer carriage, and when I look at the bore again, and how tall the wheel is based on the midshipman standing nearby, it is certanly a light boat howitzer. I should have remembered, anyway, from looking at records in the Nat. Archives, the light smoothbore howitzers were what was issued for training at the Naval Academy, they had a number of that model after the CW.
Is the officer standing to the right regular Army, helping Midshipman drill in preparation for sea duty with the Dahlgren?
My friend in CMH identifies the person with sword as a midshipman officer. His rank insignia on sleeve appears to be that of midshipman ensign-one gold stripe with loop to indicate it is a midshipman rank, not commissioned officer.