Brian, it is usual to have indexes of deaths at sea, rather than where they were buried, which presumably is included in the actual cert. For example, the Miscellaneous Records of the GRO (England and Wales) include Marine Births & Deaths, from July 1837.

Parish registers include people who drowned. e.g. Maughold reg. has:
Jon XTIAN
Jon MOORE
Jon CROW
Syl CASEMT
Phil GARRETT who all perished at sea Dec 18 1731

and many similar entries.

Sue