Hi, Robbo!
A jacket is certainly not necessary on the PG. Yet, many of us (men) chose to bring and wear one at times (if we have one we like) simply because we think we look better in it. (We are probably deceiving ourselves here!) So, bottom line, the "dress code" is casual requiring only slacks and a sports shirt for men. Anything beyond that is a matter of personal preference for the man, and not part of the ship's dress code. In other words "wear what you feel good in and feel you look good in."
Thanks,
Richard :) :) :)
