Sardine
Common55.2%ObjectiveOptionalAnywhere
Utilize a boat
Many Dover/Calais (or nearby) jobs will either need you to take the channel ferry (for this achievement), or the Eurotunnel for Choo-Choo, just pick one of those. To be unambiguous, this is the English Channel between the UK and France.
Starting your career in the south of the UK helps with this - as you'll then get those jobs pretty much immediately in the career.