The actions of terrorist groups present a threat to the national security of an increasing number of countries around the world, and the members of NATO (English: North Atlantic Treaty Organization) are especially exposed. Croatia is a member of NATO and the European Union, whose armed forces and other institutions are involved in various peacekeeping missions and other activities, which, to a certain extent, increases the possibility of possible terrorist attacks. Recent events increasingly confirm this since the beginning of Russia's military aggression against Ukraine. Terrorist activities affect security, a prerequisite for the protection of citizens, property, traffic, the economy, and other social activities. An underwater explosion can be the result of terrorist activities, war activities or economic and other activities. Terrorists use explosives and various improvised explosive devices in their attacks due to low cost and ease to make.. Terrorist activity, as an xample of asymmetric warfare, is more and more frequent. Hence, the study of the effects of underwater explosions on underwater structures is a scientific area of particular interest for national security. Studying the effect of the explosion on underwater buildings, considering the necessary measures that will enable protection against damage and demolition, on the one hand, and the other hand, considering the possibility of achieving as superficial damage and repair as possible. These analyses are very significant and necessary in spatial planning, design, and construction, depending on the objectives of potential military actions in the field of operations as well as possible terrorist actions. One of the possible targets of the mentioned attacks is coastal and hydro-technical buildings and infrastructure, which are vital buildings and the destruction of which can result in the cutting of communication lines in the area of operations, the cessation of electricity and gas supply, and significant material damage and a large number of human casualties (soldiers and civilian). The focus of the doctoral dissertation is on the preparation of numerical models and the implementation of numerical simulations of an underwater explosion in the LS Dyna computer program for research implementation. Numerical simulations were performed for several groups of models, where explosives and water were discretised with finite elements of different appropriate sizes. Experimental investigations of underwater explosion effects are pretty complex, dangerous, and expensive, so computer numerical simulations are carried out, which enables the intensive development of computers and computer programs and numerical models, which result in more acceptable results and can significantly reduce the number of necessary experimental investigations. Furthermore, the modelling procedure is described, and numerical models of the underwater explosion in the LS Dyna computer program are presented. Then, underwater explosion research was conducted using numerical models, and the models were validated. The obtained research results were presented and analysed. Then, recommendations were given for the practical use of a computer program for the engineering calculation of underwater explosion effects and guidelines for further research. The rapid development of computer technology and computer programs enables progress in the study and solution of this very complex and complex task on very complex numerical models with the application of increasingly complex and powerful computer programs. Further research should be directed towards targeted experimental studies to verify the obtained results confirm the numerical models and further develop numerical models and computer programs to implement a complete numerical simulation of an underwater explosion.