The ARMUS system presents a new approach for observation of the hull of large marine vessels providing fast, but at the same time very accurate and reliable method for underwater inspection, detection and identification of various threats. ARMUS is magnetic tracks based mobile robot capable to stick-on and move along iron surfaces and operate in extreme environmental conditions, including under the water surface. Among the main advantages of the ARMUS system is its ability to reduce significantly the time for hull inspection by providing in process estimation and assessment of the hull while the ship is in motion. This advantage reduces significantly the standby time of the ships when inspection of the hull is required before entering a port. On other hand the in process inspection allows the vessels to be observed continuously during the voyage. Another important advantage of ARMUS is its capability to stay underwater as long as it is necessary. The system is powered and controlled directly by the vessel it inspects and may operate in long cycles without maintenance and recharge.