With the huge proliferation of sensory applications, the Internet of Things (IoT) is promising connectivity capacity far beyond the conventional computing platforms, with an ultimate goal of connecting all everyday objects. Sensory applications in the marine environment are foreseen to be an integral part of this connected world, forming the Internet of Marine Things (IoMaT). While some efforts that aim to establish network connectivity in such a sparse environment exist, securing these networks is still an unreached goal. This paper introduces a secure Mobile Ad-hoc/Delay Tolerant routing protocol (S-MADNET) for the marine environment over VHF equipment available on the majority of ships. The proposed secure network is designed to use the existing Automatic Identification System (AIS) that ships use for positioning and navigation aid. An IoMaT routing module that forwards marine sensory data using the proposed secure protocol is also presented, taking the AIS system level considerations into account. Furthermore, a new AIS message format with IoMaT support is proposed that accommodates the requirements of the secure routing protocol. Evaluation results show that the proposed S-MADNET routing protocol outperforms its counterparts in terms of packet delivery rates and packet duplication rates, while maintaining data security.