FindMyPast has records of seamen and passengers who died whilst on board a British registered vessels.
The Deaths at Sea register should tell you, the name of the ship, her official number, cause of death, place of death. This information usually comes from the vessels logbook. When the vessel returns to a UK port details from the logbook are transcribed by Registrar General of Shipping and Seamen, who in turn passes it to the Registrar Generals for England and Wales, Scotland and Ireland. Which ever is appropriate.
Unless there was doctor on board, no Death Certificate was issued. A shorter copy may be obtained from GRO which is usually a copy of the details contained within the register compiled by RGSS.