The problem of controlling Underwater Vehicle-Manipulator Systems (UVMSs) is addressed in this paper. The serial-chain structure of such systems is exploited in order to formulate a control law based on the Virtual Decomposition approach. Each single body in the system is then controlled with a suitable adaptive control law based on a minimum number of parameters. The proposed approach results in a modular control scheme which simplifies application to multibody systems with a large number of links, reduces the required computational burden, and allows efficient implementation on distributed computing architectures. Furthermore, the occurrence of kinematic and representation singularities is overcome, respectively, by expressing the control law in body-fixed coordinates and representing the attitude via the unit quaternion. To show the effectiveness of the proposed control strategy, a full-degree-of-freedom simulation case study is developed for a vehicle in carrying a six-degrees-of-freedom manipulator.