Understanding of hydrodynamics involved in the ow around tidal turbines is essential to enhance their performance and resilience, as they are designed to operate in harsh marine environments. During their lifespan, they are subjected to high velocities with large levels of turbulence that demand their design to be greatly optimised. Experimental tests have provided valuable information about the performance of tidal stream devices but these are often conducted in constricted umes featuring turbulent ow conditions dierent to those found at deployment sites. Additionally, measuring velocities at prospective sites is costly and often dicult. Numerical methods arise as a tool to be used complementary to the experiments in investigations of tidal stream turbines. In this thesis, a high-delity large-eddy simulation computational approach is adopted and includes the immersed boundary method for body representation, due to its ability to deal with complex moving geometries. The combination of these numerical methods oers a great balance between computational resources and accuracy. The approach is applied and validated with simulations of vertical and horizontal axis tidal turbines, among other challenging cases such as a pitching airfoil. An extensive validation of predicted hydrodynamics, wake developed downstream of the devices or structural loadings, outlines the accuracy of the proposed computational approach. In the simulations of vertical axis tidal turbines, the blade-vortex interaction is highlighted as the main phenomenon dominating the physics of these devices. The horizontal axis tidal turbine is simulated under dierent ow and turbulence intensity conditions, in both at and irregular channel bathymetries. This thesis seeks to assess and enhance the performance, resilience and survivability of marine hydrokinetic devices in their future deployment at sea.