The Confederate States Navy regulations, Chapter 25, Atricle 6 [relating to the CS Marines on vessels], clearly states, and I quote: "They are not to be diverted from their appropriate duties, or called upon to coal ship or work as mechanics, except in case of emergency."
Although these were regulations for the Confederate Marines, they would have been exactly the same for the United States Marines, as well.
It would have to have been an extreme emergency for the Captain of the vessel to enlist the assistance of one of his Marines as his clerk, especially since the USS SUSQUEHANNA was a larger vessel, and surely would have had one of her Naval crew who was proficient in the duties of a clerical assistant.
I can only assume that Bassler was called upon to perform these duties during a period where the original clerk was ill or unable to perform his duties for some other reason, and no one else was available to do such duties, and that it was only for a temporary period.