For the time being several simulation systems are developed to simulate underwater activities. Two projects, in which FOI is involved in the development, are SaSS-UV and UV stridssimulator. Even though these projects partly are similar to each other they differ in several aspects. The UVstridssimulator is a cooperation project with other organizations and it is built on HLA. SaSS-UV is an FOI internal project which is based on using the simulation framework FLAMES. In this report the benefits and disadvantages of using the architectures HLA and the simulation framework FLAMES for simulation of underwater activities are investigated. The comparison between the architectures is made by exploring their abilities for several different criteria. The aim of this report is not to compare the projects UV-stridsimulator and SaSS-UV, but rather to compare their underlying architectures. Even though the architectures qualitatively differ in several aspects, it is not possible to decide which one is better. Hence, the report is focused on indicating in which applications one architecture is better suited than the other.