I am not sure how much help I can be finding information on your ship captain. I know that slaves & free blacks often manned and sometimes captained or piloted small boats during the war, but I am at a loss to know where to search for information.
I do have the names of a couple more slaves serving in the 41st MS Infantry as laundresses. Besides Peter in Company B, there also were Adam and Remous in Company K. Both men were only shown present for the roll of Mar-Apr 1864 while Peter was present from March to August 1864. I thought that there may be one assigned to each company, but my quick search in Fold3 only revealed these three names. The names are distinctive in the rolls since they are only identified by a single name. I remember reading somewhere that Col Bird Williams brought slaves in to do laundry for his men during the Atlanta Campaign, but I was surprised to find them on the rolls.