In this Chapter the mathematical model of UVMSs is derived. Modeling of rigid bodies moving in a fluid or underwater manipulators has been studied in literature by, among others, [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12], where a deeper discussion of specific aspects can be found. In [13], the model of two UVMSs holding the same rigid object is derived. A short introduction to underwater vehicles, without manipulators, thus, is given by [14], while deep discussion may be found in [15, 16, 17].