jkneed,
this link guides you to "Metro del Mare":
http://www.metrodelmare.com/linee_g.html
I have no experience with this line.
http://www.metrodelmare.com/l-linee_03.html
LIne N. 1 has a stop at Naple -Beverello (That is just beside the cruise terminal).
Maybe one of the members here has more information.
Regards
Uwe