In the US Navy, 166X is the designator for a Strategic Sealift Officer (SSO). SSOs serve in traditional Mariner positions on civilian ships, including oceans, lakes, and inland rivers. They have the qualifications to command and/or provide engineering support for a ship.
To become a SSO, you must:
Complete three Naval Science courses: Introduction to the Strategic Sealift Program, Naval Science for the Strategic Sealift Officer, and Leadership and Ethics
Participate in a Naval Science Laboratory
To become a SSO, you must have previously been a:
Merchant Marine
Public Affairs Officer
Fleet Support Officer
Human Resources Officer
Reserve Recruiting Officer