There are a few mariner resources here:
www.opc-cornwall.org/Structure/resources.phpeg:
British Army & Navy Baptisms in Cornwall 1814-1879
British Army & Navy Marriages in Cornwall 1730-1919
British Army & Navy Deaths of Cornish-born
Masters & Mates Certs for Cornish-born men 1850-1927
Royal Navy Registers of Cornish Seamen's Services, 1848-1939 and Naval Officer & Rating Service Records 1802-1919 (large file)
Merchant Seamen deaths WW2
Royal Naval Reserve Casualties 1730-1960 (Cornish Districts)
Also searching the Royal Cornwall Gazette might be worth a look, if you know the seaman's name
Here's an example of one transcribed on opc-cornwall.org, but you would probably be better off using British Newspaper Archive (maybe FindmyPast?) to research all the entries in the Gazettes
https://www.opc-cornwall.org/deaths/gunwalloe_death_unidentified_seaman_1869.pdf