For general moving object (GMO) problems, when mass density of the moving object(s) under coupled motion is less than that of fluid, the existing explicit GMO method in FLOW-3D® often fails due to stability difficulties. In this work, an implicit GMO method was developed and incorporated into FLOW-3D®. The main difference between the implicit and the explicit GMO methods is that in each computational cycle, or time step, the former calculates the object motion and the fluid flow iteratively while the later calculates them separately. Unlike the explicit method, the implicit approach imposes no limitations on mass density of the moving objects. Tests show it possesses good stability for density of moving objects as low as 0.1% of that of fluid. Close matches between the computational and experimental results were obtained for simulations of a light boat under coupled motion in water stream.