Depending on whether the non-resident
alien is working for a United States registered yacht or not.
Of course immigration issues are paramount here. Provided that
the necessary visa or passport.
a) Nonresident aliens working
for a United States registered yacht must follow all United
States tax rules. Tax Treaties are controlling for each non-resident.
Non resident aliens very often do not have to pay any taxes
depending on their treaty based exceptions.
b) Non resident aliens working
for a foreign flag vessel do not need to pay taxes if not in
United States waters. If you find yourself in United States
waters or land you will need to follow your tax treaty again