Recent methods for in-situ monitoring and actuation through modern fixed and mobile marine technologies are becoming nowadays a very useful tool that helps scientists and stakeholders to monitor, sample, and actuate at unprecedented locations in the ocean. These systems have an ever-expanding range of applications. They can operate both in the surface and in the deep waters to carry out scientific missions like profiling, mapping, and geological and geophysical surveys; commercial missions like inspecting assets and infrastructure; exploration missions for minerals and seabed mining; and public authority missions like surveillance and search and find (like finding aircraft wreckage). Monitoring networks of cooperative underwater marine robotics can expand the spatiotemporal measuring capabilities, resolution, and reach, providing the suitable infrastructure to manage data flow from robotic technologies equipped with innovative sensor payloads. The author’s experience in the design and construction of cabled seabed observatories is taken as background, and this chapter will give an overview of different types of marine technologies, the challenges of navigation, geolocalization, different types of missions, payload, and applications where the different devices and marine equipment arrive where humans do not.