The current work presents a novel way to evaluate the interaction effects in a diffuser-augmented hydrokinetic turbine (DAHT) under the terms presented on the generalized actuator disc theory described by Jamieson (2011). Transient RANS CFD methods are employed to obtain the performance, thrust, and average flow speeds at the turbine plane in simulations performed in three comparable cases: bare turbine, bare diffuser and diffuser-augmented turbine. After validating the bare turbine case comparing numerical results against the experiments presented by Fontaine et al. (2013), the axial induction factors are obtained for the three cases based on the average speeds of the turbine plane. The analysis of the results shows the importance of considering rotor-diffuser interaction for the design of diffuser-augmented devices, and that these interaction effects are just as relevant as the bare diffuser and the bare turbine characteristics.