The concept of maritime IoT was originally developed by the United Nations' International Maritime Organization under the name e-Navigation for the harmonization and digitization of marine navigation information and operation, and supporting a broad variety of maritime services. As with any other IoT application, communication, in particular the MTC, is the key to the materialization of this concept. However, maritime MTC faces many practical challenges rooted in maritime environments, service requirements, and the radio spectrum. This article addresses these requirements and challenges as well as the solutions with a focus on the network architecture, air interface, and radio spectrum of such a unique communication system.