Offshore floating nuclear power plants are playing an increasingly important role because of their safety and flexibility. However, there is no consensus on the characteristics of thermal-hydraulic parameters and the mechanism of bubble behavior in the reactor core under marine conditions. It is urgent to conduct in-depth research on the two-phase flow characteristics in the core from a microscopic perspective. Therefore, in this paper, a numerical model of the subcooled flow boiling in the core under marine conditions is constructed by using the interface-tracking technique. The characteristics of the microscopic two-phase flow and bubble behavior in inclined and vertical subchannels are analyzed. The differences in macroscopic two-phase parameters, such as transverse velocity, void fraction, and bubble adhesion ratio, are compared between the inclined condition and the vertical condition. The distribution characteristics of each parameter in the whole and local fluid region are analyzed in the inclined condition. The influence of the inclined state on the transverse flow pattern and phase distribution in the subchannel is discussed. Under inclined conditions, the two-phase flow characteristics are affected by the inclination angle of the channel and the orientation of the heated surface (the positive and negative of the inclination angle). The component of buoyancy on the cross-section is the fundamental reason for the change in two-phase flow characteristics. With the increase of the inclination angle, the component of buoyancy on the cross-section increases. And the difference in the two-phase flow characteristics between the inclined condition and the vertical condition also increases. This research is of great significance in revealing the boiling two-phase flow phenomenon and the mechanism of bubbles behavior in the core under marine conditions. The results of the study provide a great guidance to improve the safety and economy of offshore floating nuclear power plants.