Dear Mr. Wright,
Thank you for posting your request on History Hub!
I believe you may be best served to reach out to our College Park unit, to inquire if they might have any dispatched from William H. Seward regarding the loss of the emigrant vessel William Nelson. Please note that this was not a vessel of the US naval forces, so it did not have the USS (United States Ship) designation.
For more information about these records, please contact the National Archives at College Park - Textual Reference (RDT2) at firstname.lastname@example.org.
In researching the fire and sinking of the William Nelson, I was able to locate the article, republished in the New York Times of the incident dated July 27, 1865.