Abstract
This work presents the development of low-cost, low-power, and disposable marine monitoring technologies designed to support oceanographic studies in remote and extreme environments. These platforms were initially targeted for underwater volcanic locations but offer a broader application potential for ocean research. Three main technologies were developed and tested: underwater monitoring probes for real-time water quality assessments in geothermal springs, deep-sea probes for vertical ocean profiling and autonomous drift-phase monitoring, and a surface buoy for rapid-response environmental monitoring. Field deployments in diverse locations, including the Ponta da Ferraria (S. Miguel, Azores), Banco D. Joao de Castro (Atlantic Ocean), and the Cumbre Vieja volcanic eruption site (La Palma, Canarias), demonstrated the operational feasibility of these systems. Despite challenges in deep-sea operation and deployment conditions, the results highlight the potential of these platforms for scientific studies, environmental monitoring, and emergency response. Their adaptability and modularity make them valuable tools for a wide range of oceanographic applications beyond their initial focus. Ongoing efforts to improve marine communication reliability, sensor integration, and resilience to extreme ocean conditions hold the potential to further expand the role of these technologies in marine exploration.

Abstract
Measuring water motion is essential for oceanography, coastal engineering, and marine environmental monitoring. A wide range of sensing technologies is used to quantify water velocity, wave motion, and flow dynamics, each suited to specific spatial and temporal scales. This paper presents a comprehensive review of modern sensor technologies for marine flow measurement, covering mechanical, electromagnetic, pressure-based, acoustic, optical, MEMS-based, inertial, Lagrangian, and remote-sensing approaches. The operating principles, strengths, and limitations of each technology are examined alongside their suitability for different environments and deployment platforms, including moorings, buoys, vessels, autonomous underwater vehicles, and drifters. Special attention is given to rapidly advancing fields such as MEMS flow sensors, multi-sensor fusion, and hybrid systems that combine inertial, acoustic, and optical data. Applications range from high-resolution turbulence measurements to large-scale current mapping and wave characterization. Remaining challenges include biofouling, performance degradation in energetic shallow waters, uncertainties in indirect velocity estimation, and long-term calibration stability. By synthesizing the state of the art across sensing modalities, this review provides a unified perspective on current technological capabilities and identifies key trends shaping the future of marine flow measurement.