Ducted wind turbines have been investigated for decades to improve the efficiency of renewable energy production. Previous research has attempted to obtain a competitive commercial design without any relevant success. However, there is not enough evidence to disregard the potential advantages of the ducted concept for water turbines. In the ocean and marine energy industry, most prototypes are in a relatively early stage of technical development, which opens up an opportunity to re-examine alternative designs through numerical computation. This paper aims to describe the numerical performance of a unidirectional duct coupled to a 30 cm diameter horizontal axis water turbine using computational fluid dynamics. Two different approaches were used to assess the system’s performance. In the first approach, the turbine was modeled as a momentum source region according to the linear momentum actuator disk (LMAD) theory. At a flow velocity of 0.7 ms−1, the ideal ducted turbine produced 1.42 times more power than its ideal bare counterpart. In the second approach, a blade geometry was designed using the standard blade element momentum (BEM) method, and the fluid interactions were simplified to a steady state by applying a moving reference frame (MRF) simulation model. For this configuration, the ducted turbine produced 1.48 times more power than its bare counterpart. An additional analysis was performed based on the power densities, and it was found that the use of a duct does not represent a real power increase in terms of the efficiency. However, several alternative advantages for the exploitation of ocean currents with low energetic flow conditions were identified and discussed.