The maritime environment still represents an unexploited potential for modelling, management and understanding of mobility data. The environment is diverse, open but partly ruled, and covers a large spectrum of ships from small sailing-boats to super tankers which generally exhibit type-related behaviours. Similarly to terrestrial or aerial domains, several real-time positioning systems, such as the Automatic Identification System (AIS), have been developed for keeping track of vessel movements. However the huge amounts of data provided by these reporting systems are rarely used for knowledge discovery. This chapter aims at discussing different aspects of maritime mobilities understanding. This chapter enables readers to, first, understand the intrinsic behaviour of maritime positioning systems and then proposes a methodology to illustrate the different steps leading to trajectory patterns for the understanding of outlier detection.