My g.grandmother's son from her first marriage drowned in the North Sea on 17 February 1906, and although I have all the info, including his occupation and home address, which someone researching Hull fishermen compiled from all the ships' logs, he is not registered anywhere as far as I can see, and by 1908 records were much better than the 1870s, so it seems to me that a lot of deaths at sea were either never officially recorded or recorded in the country in whose waters they drowned, which if you don't know where it was is not very helpful.
Lizzie