Three-dimensional numerical simulations of hydrokinetic flapping-foil turbines with fully-prescribed motions are carried out using a Delayed Detached-Eddy Simulation (DDES) approach. The objective is to assess the efficiency and the wake recovery rate of flapping-foil turbines equipped with either end-plates or detached end-plates. The power coefficient and the mean streamwise velocity recovery in the wake of a turbine with different geometries of end-plates and detached end-plates are analyzed and the results are compared to a high-efficiency reference turbine without end-plates. We show that the turbine efficiency is significantly increased when detached end-plates are used. Indeed, the best configuration simulated in this work has an efficiency value that is 21% larger than that of the reference turbine. Moreover, the main physical mechanisms affecting the wake recovery rate are the same whether or not detached end-plates are used. Interestingly, for a turbine with small detached end-plates, the mean streamwise velocity recovery in the wake is similar to that of the reference turbine, while the efficiency is increased by 15%. This suggests that such detached end-plates could be quite beneficial for flapping-foil turbines operating in turbine farms.