The technology development in the areas of uncrewed and autonomous systems is creating many opportunities for use, both in the civil and the military realm. The technology itself has become faster and more precise in the situations it is exposed to compared to a human in the same situation, bringing on the discussion of where and when these systems are acceptable to use. This study investigates how the use of autonomous systems in naval applications are affected by legal, for example UN Convention on the Law of the Sea, and ethical concerns, such as meaningful human control, when performing a mission at sea. The aim is to support development and implementation efforts. Legal and ethical aspects are applied to two hypothetical cases using small autonomous underwater vehicles to illustrate challenges. The challenges are often connected to trust in the system and the accountability for its actions, making it difficult to see the benefits of using such systems, resulting in the benefits being overridden by possible negative effects. Therefore, it is necessary to have a balance between trust and risk and a balance between technology opportunities and governance regulations, where the two opposites must evolve together for a reliable system.