Roughly two-thirds of our planet is covered with water, and so far, the oceans have predominantly been used at their surface for the global transport of our goods and commodities. Today, there is a rising trend toward subsea infrastructures such as pipelines, telecommunication cables, or wind farms which demands potent vehicles for underwater work. To this end, a new generation of vehicles, large and Extra-Large Unmanned Underwater Vehicles (XLUUVs), is currently being engineered that allow for long-range, remotely controlled, and semi-autonomous missions in the deep sea. However, although these vehicles are already heavily developed and demand state-of-the-art communication technologies to realize their autonomy, no dedicated test and development environments exist for research, e.g., to assess the implications on cybersecurity. Therefore, in this paper, we present XLab-UUV, a virtual testbed for XLUUVs that allows researchers to identify novel challenges, possible bottlenecks, or vulnerabilities, as well as to develop effective technologies, protocols, and procedures.