This paper investigates Nonlinear Time-Varying Mixed Lubrication (NTVML) performances of a Coupled Spiral Microgroove Water-lubricated Bearing (CSMWB), which consists of both thrust and journal microgroove bearings, using a developed numerical model. A 5-degree-of-freedom (5-DOF) dynamic model of the CSMWB is fully coupled with a transient mixed lubrication model considering mass conservation cavitation. The validity of the present model is supported by the corresponding experimental evidences in terms of hydrodynamic load capacity and friction coefficient. Based on the model, the evolutions of the transient interfacial behaviors, such as hydrodynamic pressure, elastic-plastic contact pressure, cavitation distribution etc., under nonlinear external excitations are revealed, and the significant role of the mass conservation boundary condition on the NTVML performances in such bearings is highlighted. Subsequently, an extensive parametric study is performed to determine the optimal design of such bearings. Numerical results indicate that a relatively large spiral angle of the thrust bearing has potential to reduce the transient asperity contact for both the journal and thrust bearings. Moreover, a relatively small microgroove width ratio can improve the NTVML performance of journal bearing, while it increases the transient asperity contact of the spiral microgroove thrust bearing. Furthermore, this paper demonstrates the applicability of the CSMWB in rim-driven thrusters and provides the recommended parameters for the optimal performance.