An optimum utilisation of the energy available in the ocean could meet our global energy demands. The implementation of tidal stream technologies is striving faster compared to other offshore technologies, aside from the offshore wind sector. Highly energetic tidal energy streams with peak flow velocities in the order of 2.55.0 m/s are limited around the globe and therefore, the sector may be constrained to a few worldwide locations. To overcome this limitation, attention has been drawn to the exploration of complementary flow streams with lesser energetic flows. This work thus intends to compare the feasibility of harvesting hydrokinetic energy using horizontal axis turbines from two different sites: a typical tidal stream site in the North of Scotland and an ocean current site in the Mexican Caribbean, characterised by developing slower but more stable flows. The viability of using ocean currents against tidal stream sites is analysed in terms of annual energy output, capacity factors and an initial estimation of the levelized cost of energy which considers the size of the turbine and rotor characteristics. As expected the annual energy produced by tidal devices is overall greater than that provided by marine currents but the capacity factor achieved with a typical tidal turbine can be in the order of 44% whereas the capacity factor calculated for a turbine operating in the Yucatan current can achieve factors in the order of 77%, giving confidence that the development of marine projects in lesser energetic f lows may be the next step forward to advance this sector. This research also proposes the optimal turbine diameter to reduce the levelised cost of energy for a marine converter installed in a tidal or an ocean current site.