Hi, Not sure what a private yatch is but a yacht and a Cruise Ship do often employ Marine Techs they are generally known as Engineers on yachts and Engineering Officers on Cruise and Commercial ships. Private Yachts are more liberal with the licensing requirements whereas Commercial Vessels have a Safe Manning Document and a set of rules and regs to follow with the occasional dispensation granted in rare cases. You can probably work on a Private Yacht right now but there are a number of courses you have to do to legallly serve on a commercial vessel look up STCW 95 Basic Requirements to get an idea.