Investigations on dynamics and motion control for a crablike robot in floating gait are conducted, in order to achieve the waypoint-tracking control of the leg-paddle coupling crablike robot disturbed by unknown current and flow. Firstly, dynamic model of the robot subject to complex underwater environment is established with consideration of the effects of the gravity, the buoyancy, the thrust produced by the swimming leg and the hydrodynamic force. On this basis, a sliding mode variable structure controller based on exponential approach law is designed. The phase difference between the flapping motion and the feathering motion is taken as the controlled variable to achieve the yaw rate control. And the globally asymptotic stability of the control system is proved by the Lyapunov direct method. Simulations and experiments of single and multiple target tracking are carried out, and the results show that the presented controller has strong waypoint tracking ability and robustness against the variations and disturbances of the system dynamics.