Marine current turbines can provide predictable renewable power, but their performance at moderate current speeds depends strongly on the flow delivered to the rotor. This study numerically evaluates an open horizontal-axis marine current turbine and two flanged diffuser configurations with short and long expansion sections. Three-dimensional geometries were created in SOLIDWORKS and analysed in ANSYS CFX using a steady-state Reynolds-averaged Navier-Stokes approach with the shear-stress-transport turbulence model. The turbine employed a NACA 63-4XX blade profile, a rotor radius of 2.2 m, a hub ratio of 0.2, and a design tip-speed ratio of 5.1. The inlet current speed was specified as 3 m/s. The reported pressure drop increased from 1475.5 Pa for the open turbine to 5599 Pa for the short diffuser and 9802 Pa for the long diffuser. Flow contours indicate that the flange generates a downstream low-pressure region and vortex structures that draw flow towards the rotor. Within the limits of the steady numerical model, the long-expansion configuration produced the largest pressure difference and therefore the strongest indication of available pressure-energy conversion. Further work should quantify torque and power coefficient, establish mesh independence, validate the model experimentally, and assess cavitation and bidirectional tidal operation.