Due to challenges, such as information collection in huge water, propulsion at different scales and manipulation in extreme fluid environments, wide application of unmanned underwater vehicles (UUVs) is limited. In this paper, an alligator modular robot, composed by 14 small robots, was inspired from the locomotion and morphologies of alligators and microorganisms. With the undulatory body and beating legs, the assembled large robot was able to maintain fast and maneuverable swimming for tasks executions at large Reynolds number. With a biologically optimized body aspect ratio (3.47), small modular robots achieved effective propulsion in viscous fluid. Facilitated by a dissembling system, small robots were automatically released from the large robot frame and swam around with independent actuation systems. Hydrodynamic model for the large and small robot were formulated, based on which the propulsion for the modular robot in large and low Reynolds numbers (Res) were simulated. Experiments were conducted to compare with the simulation results to further validate the proposed modular robot design.