Research on autonomous vehicles has been a key area of concern especially in the last two-three decades. Underwater vehicles took their share in such studies. In addition to single remotely-controlled and autonomous underwater vehicles, ongoing research deals with construction of coordinated missions to be performed by groups of such vehicles. In this study, which can be considered as a condensed review of the underwater robot swarms, we try to summarize the challenges and practical issues in this area. In addition, we try to illustrate the advantages of a swarm formation with a basic case study