A second engineer or first assistant engineer is a licensed member of the engineering department on a merchant vessel. This title is used for the person on a ship responsible for supervising the daily maintenance and operation of the engine department. They report directly to the chief engineer.
A sample of a credential given by the US Coast Guard to all officers, containing what vessels they are certified to work aboard
A licensed mariner is a sailor who holds a license from a maritime authority to hold senior officer-level positions aboard ships, boats, and similar vessels. Qualification standards for licensed mariners are universally set by the STCW Convention adopted and promulgated by the International Maritime Organization (IMO), while the licenses of individual sailors are issued by the delegated maritime authorities of the member states of the IMO; these may vary in the details of the implementation, including the government agency responsible for licensing and the local names of the grades and qualifications in each particular country.
A sample United States Merchant Marine license issued by the United States Coast Guard in 2006
Mariners (Captain, first officer and second officer) at the controls of the Kristina Regina
