## 1 Introduction  
Coastal marine systems are particularly vulnerable to the effects of human activity attendant on industrial, tourist and urban development. Information and communications technologies offer new solutions for monitoring such ecosystems in real time. In response to this demand for technology, the last ten years have seen the emergence of various initiatives, from simple case studies to complex coastal observation systems designed to monitor the marine environment. These systems are composed of sensor nodes, frequently wireless, which transmit data to a sink node, in real time, on a number of physical, chemical and/or biological measurements (temperature, pH, dissolved oxygen, salinity, turbidity, phosphates, chlorophyll, _etc._). In this context, Wireless Sensor Networks (WSNs) [1,2] offer a new paradigm for oceanography, as in many other disciplines such as precision agriculture, environmental, engineering, WSNs are a type of autonomous, self-organized _ad-hoc_ Wireless Personal Area Networks (WPAN) composed of tens, hundreds or even thousands of smart low-rate battery-powered sensor nodes (motes). In a WSN, sensor nodes normally consist of a processor, a radio module, a power supply and one or more sensors mounted on the more itself or connected to it. The processor controls all the node's functions such as access to sensors, control of communications, execution of algorithms, battery saving, energy source management, _etc._  
The design, implementation and deployment of a WSN for oceanographic applications poses new challenges different to the ones that arise on land, as the impact of the marine environment on the sensor network limits and affects their development. The following are some of the most important differences:  
* The marine environment is an aggressive one which requires greater levels of device protection. * Allowance must be made for movement of nodes caused by tides, waves, vessels, _etc._
* Energy consumption is high since it is generally necessary to cover large distances, while communications signals are attenuated due to the fact that the sea is an environment in constant motion. * The price of the instrumentation is significantly higher than in the case of a land-sited WSN. * There are added problems in deployment of and access to motes, the need for flotation and mooring devices, possible acts of vandalism, and others. In spite of all these drawbacks, various case studies of monitoring of marine ecosystems using WSNs can be found in the literature [3-14]. What these solutions have in common is that they are largely designed and implemented _ad-hoc_ (buoys, electronics and software), and oceanographic sensors and some other components are normally the only elements acquired from third parties. In the scientific literature we can distinguish two broad categories of marine wireless networks depending on the data transmission medium that they use: WSNs based on radio frequency (RF) aerial communications (hereafter called Aerial-WSNs or A-WSNs) and Under-Water Acoustic Sensor Networks (UW-ASNs). A UW-ASN [15] consists of a variable number of sensors and vehicles that are deployed to perform collaborative monitoring tasks over a given area. In underwater conditions RF does not work well because radio waves propagate only at very low frequencies (30-300 Hz) and special antennas and a bigger power supply are required. The problems that underwater acoustic networks of this kind pose include: bandwidth is severely limited; propagation delays are five orders of magnitude greater than in terrestrial radio frequency channels; higher bit error rates and temporary losses of connectivity; limited battery power because solar energy cannot be used, and so on. UW-ASNs are the best solution for viable oceanographic monitoring at great depths entailing the use of Autonomous Underwater Vehicles(AUVs) equipped with underwater sensors. Underwater networking is a relatively little-explored area although experimentation with underwater communications has been going on for several decades.