It will be almost impossible to tell which Charles Johnson is the correct one in the available records. You will need at least the name and time period of a ship he served on to narrow down the rew lists. Without his birth date and the location, it also increases the difficulty. But here's some places to both obtain records and tell you how to conduct your search.

British National Archives
http://www.nationalarchives.gov.uk/records/looking-for-perso...

Helpful hints for tracing a merchant sailor-
http://www.rmg.co.uk/researchers/library/research-guides/the...