The utilisation of marine current turbines with horizontal axis for electrical power production offers a sustainable and clean option to augment traditional energy technologies and enhance the expansion of renewable energies in the marine field. Some prototypes of these turbines are now available and operating. The advantages of tidal currents for renewable energies when compared with wind resources are that sea water is about 800 times denser that air (and the extracted energy is proportional to density) and the nature of currents that results in a more predictable resource than wind resources. But these advantages set certain limitations and technical problems, some of them related with the hydrodynamic design of rotor blades since the design of a marine turbine is some kind different of the techniques used for wind turbines. Methodologies need be established to describe the physical and operational performance of the turbines, allowing their design to be investigated and performance evaluated. This paper describes the hydrodynamics of rotor design that is projected in order to extract the maximum power from the tide. The blade element momentum theory is used for the rotor modelling, different aspects and limitations of this approach when applied to marine rotor design are commented, and examples are presented.