This page explains how to start the sidequest to save Marbule in Chrono Cross: The Radical Dreamers Edition.
After receiving the Fiddler Crab from the Sage, you can enter the Dead Sea. However, before departing the S.S. Zelbess, there’s a very important thing you should do.
As Nikki mentioned, he wants to see you on his ship. Head for the main deck, where the deck chairs are. Climb the ship’s mast towards the left and ask to use the pulley lift at the top. This will take you to Nikki’s ship, the Magical Dreamers.
Enter the door near where you arrive. Nikki will discuss with his crew a plan to save Marbule from the ghostly invasion. He’ll then ask for your help with the plan. As far as we know, it doesn’t matter what you choose (you simply need to be present during the plan), but you should agree to be safe.
Everyone will then leave to make preparations, besides yourselves and Irenes. At this point, you can recruit Irenes.
For the time being, leave the S.S. Zelbess when you’re ready.