The purpose of this master thesis is to investigate how alarm systems and their visualizations, as well as signals affect work situations and decision making processes aboard vessels with water jet propulsion. Furthermore, this study aims to identify important aspects of the perception of alarm systems and which aspects that are more or less affected by stress, lack of information as well as audible and visual signals. To investigate this, the theory of Situation Awareness and the theory of Naturalistic Decision Making were used to make an analysis of which aspects that were more or less important for the users. The study found that the alarm system affect users a lot in their daily work. The aspect that was considered particularly important when designing a new alarm system was to have the users' everyday life in mind. Alarm systems that are based on guesses or what one thinks are appropriate for different users rarely turn out to be correct. Therefore, in order to design a correct alarm system, this situation should be taken into consideration. Other aspects that arose were the need to reduce false alarms and ordinary alarms in order to minimize the number of elements that can stress or disturb the users. In addition, audible and visual signals should not interfere more than necessary to call attention from the user. Finally, the thesis come to a conclusion with suggestions for a new alarm system based on the aspects mentioned above.