The ferry's delay is caused by a monstrous creature lurking in the nearby cave, the 'Waterfall Cave'. Head to the cave entrance, which is just a short walk from Port Prospect. This cave is relatively small but introduces some new enemy types, such as the Hammerhoods and Bubble Slimes. Bubble Slimes can inflict poison, so make sure you have Antidotal Herbs or Jessica's Squelch spell ready.
Navigate through the cave, collecting any treasure chests you find. The path is fairly linear. At the deepest part of the cave, you'll encounter the source of the trouble: a large, aquatic monster known as the 'Khalamari'. This is your next boss battle. Khalamari is a formidable opponent with strong physical attacks and a multi-target attack. Focus your attacks on it, using Jessica's Frizz spells for damage and the Hero's healing spells (Heal) or Medicinal Herbs to keep your party's HP up. Yangus and the Hero should be your primary damage dealers. After defeating Khalamari, the path to the ferry will be clear. Return to Port Prospect and speak to the ferry captain to arrange your passage to the next continent.