High-intensity rains affected two villages in the region of Tana Toraja
Singapore police, who manage the island nation's coastguard, did not comment immediately on the incident.
Piracy has fallen in the busy sea lane that runs past Singapore, Malaysia and Indonesia in recent years amid heavier policing.
However, the Yonhap report said seven pirates on a speedboat attacked the CK BlueBell before dawn, leaving some crew with minor injuries.