The Oscillating Water Column (OWC) is a form of Wave Energy Converter (WEC) plant that is employed to harness the power of ocean waves. UniWave is a vented OWC type that extracts the pneumatic power solely from negative pressure drops in the chamber. Having a different flow profile compared to the conventional OWCs, there is a need for customizing a turbine design for the typical UniWave operation. The designmodification of the downstream section in a radial inflow turbine has been considered for efficiency improvements. Being the linking element between the turbine and the chamber, this section has potential for significant impact on the turbine-chamber flow interaction. Four parameters are selected which mainly affect the shapes of the downstream guide vanes and the duct, and optimization methods are employed for efficiency maximization. Guide vane curvature, radius of the duct and use of a diffuser were shown to affect the efficiency and the downstream losses. An improved downstream section in a turbine-duct geometry can improve the turbine’s peak efficiency by up to 14%, due to decreasing the pressure losses at the rotor downstream.