Maritime cybersecurity is a growing concern due to the increasing number of cyber-attacks in the sector in recent years. The complexity of the port and ship ecosystem, as well as the lack of awareness and training in the sector, pose significant cybersecurity challenges. To address these issues, in this paper we describe ICoSSiuM, a cybersecurity simulator specifically tailored to demonstrate and evaluate the impact of attacks on maritime communications. We focus mainly on the study of the Automatic Identification System (AIS). This system, which is used by ships to broadcast various information about their identity, location, speed, and destination, is highly vulnerable to various cyber-attacks. We also describe and test several attack scenarios that exploit the weaknesses of this communication system for disturbing maritime operations.