Thank you both for the quick replies.
2 of the party will be staying in Shin-yokohama for 2 nights before the cruise, while the other 2 of us will be travelling directly from Narita. We are joining the RCL, and the agent said she is not aware of any shuttles.
@BruceMuzz the shuttle from Yamashita, is it open to anyone trying to get to Diakukou Pier? Is there somewhere i can find out more details on this shuttle?