## 2 Underwater Sensor Nodes  
This section shows the papers accepted in the Special Issue focused on underwater sensor node topics. Figure 1: Special Issue Papers classification. Figure 2: Classification of the topics into levels. In [4], [PERSON] _et al._ design, build, and test a portable, watertight, and user-friendly an autonomous underwater sound recording device (USR) that monitors the underwater sound and pressure waves generated by anthropogenic activities, such as underwater blasting and pile driving. It uses two hydrophones or other dynamic pressure sensors which allow approximately up to 1 h and 55 min of data collection from both sensors simultaneously. The gain can be independently set for each sensor. They proposed two versions. The first one is a submersible model deployable to a maximum depth of 300 m and the second one is a watertight but not fully submersible model. The submersible version is appropriate for collecting long-duration measurements at depths that would require very long hydrophone cables or extension cables, while the non-submersible version is better for short duration underwater monitoring cases. Their test demonstrates that the device operates as designed and performs as well as larger commercially available data acquisition systems. Moreover, they present a case study with a prototype measuring blast pressures while investigating the effect of underwater rock blasting events while simultaneously exposing juvenile Chinook salmon and rainbow trout held in cages to the same sound and pressure waves. Device withstood operation in harsh environments, thus it is a valuable tool for collecting field measurements. [PERSON] _et al._ present in [5] the development of acoustic transmitters an underwater neutrino telescope (KM3 NeT). Firstly, they deployed an acoustic transceiver as part of the acoustic positioning system in order to monitor the position of the optical sensors which move due to sea currents. The transceiver has reduced cost, low power consumption, high pressure withstanding (up to 500 bars), high intensity for emission, low intrinsic noise, arbitrary signals for emission and the capacity of acquiring and processing received signals. Secondly, they deployed a compact acoustic transmitter array for the calibration of acoustic neutrino detection system, which is able to mimic the signature of ultra-high-energy neutrino interaction in emission directivity and signal shape. They present the successful results of the tests and measurements performed with FFR-SX30 hydrophones and a custom sound emission board. Due to the versatility of the transceiver system, this prototype could be implemented to carry out several calibration tasks related to acoustic emission in underwater neutrino telescopes. Moreover, it can be used in other applications, such as marine positioning systems or integrated in different Earth-Sea Observatories, where the localization of the sensors is an issue, especially when very directive beams are required and/or signal processing techniques are needed. The paper authored by [PERSON] _et al._[6], presents an electronic meter to measure surface seawater density based on the buoyancy force measurement carried out using a magnetostrictive linear displacement sensor. It uses an Archimedes' law (measuring the difference in displacements of a surface level probe and a weighted float, the density of the water can be estimated). They describe the operating principles of the meter, the design of its mechanical and electronic parts and its features. The displacements are simultaneously measured using magnetostrictive sensors. The hardware used is a custom electronic board, which provides a wireless connection and power supply. The wireless module and the firmware of the microcontroller can be changed, which means that the node can be easily used in underwater or above surface wireless networks. Presented laboratory and at-sea tests show the performance of a wireless density meter node using Bluetooth.