I found something which might be the same Deaths at sea book you mention?
Along the top it has "Return of Deaths at Sea, reported to the Registrar General of Shipping and Seamen under the provisions of the "Merchant Shipping Act, 1854" and "Births and Deaths Registration Act, 1874," during the month of March 1880."
I would interpret that the form is in place of a death certificate?
It has actually states "missing since 2.3.80" under Date of Death, and his name was originally written as James Barnes, but with 'r' crossed out and 'i' written above.