meaning: openly, without trickery
A trick, common to warships and pirates alike, both of which had very large crews, was to keep all but a few of their men out of sight. At long distance, someone inspecting them with a telescope might be fooled into thinking they were seeing a peaceful merchantman with a small crew, and so no threat. The devious captain kept his men low behind the bulwarks — or below the top deck; a captain with nothing to hide would keep his crew “above board.”
