Small submersible vessels have been used for years by nation states, terrorist groups, and criminal organizations to achieve operational objectives. These entities have sought to capitalize on the overriding tactical advantage of submersible technology which is stealth. For example, drug trafficking organizations in Central and South America have been routinely using self-propelled semi-submersible vessels to clandestinely transport large quantities of illicit drugs to North America. Small submersible vessels can also be nefariously used in the maritime domain to transport persons or weapons or they could be used as waterborne improvised explosive devices. Terrorists and criminals are complex adaptive adversaries and are driven to innovate when confronted with threats to their operational effectiveness. Innovation and adaption are driving these foes to leverage disruptive technology towards the development or acquisition of fully-submersible vessels. Furthermore, there is a growing population of privately owned submersibles within the U.S. that policymakers have little visibility of. Homeland security policymakers lack adequate situational awareness regarding the vulnerabilities, threats, and consequences to the maritime transportation system from the malicious use of submersibles. The core challenge for the maritime homeland security enterprise with regards to submersibles is developing effective strategies to mitigate their potential risk.