In the age of technological advancement and the constant evolution of Uncrewed Underwater Vehicles (UUVs) with ever-increasing scopes, applications, and autonomy, it is crucial to identify and manage potential risks at the design stage to ensure their safety and efficiency. Such risk management is more important, in particular, given the increasing demand for monitoring and securing critical maritime infrastructure.This paper presents a generic set of ideas of how risks can be mitigated for large and extra-large UUVs. A risk ranking matrix, a result of a Hazard Identification Study (HAZID) study for those UUVs, forms the basis for developing methodologies and systems to mitigate the identified risks and hazards. This is done for the three main systems, i.e., mechanical system, energy system, and autonomy, as well as for the orthogonal topic of cyber security. Each system and its subsystems have their risk classification and need specialized techniques to mitigate the risks to a tolerable level considering costs and probability.