In this paper, publicly available cost data are used to calculate the private levelised costs of two marine energy technologies for UK electricity generation: Wave and Tidal Stream power. These estimates are compared to those for ten other electricity generation technologies whose costs were identified by the UK Government (DTI, 2006). Under plausible assumptions for costs and performance, point estimates of the levelised costs of Wave and Tidal Stream generation are £190 and £81/MWh, respectively. Sensitivity analysis shows how these relative private levelised costs calculations are affected by variation in key parameters, specifically the assumed capital costs, fuel costs and the discount rate. We also consider the impact of the introduction of technology-differentiated financial support for renewable energy on the cost competitiveness of Wave and Tidal Stream power. Further, we compare the impact of the current UK government support level to the more generous degree of assistance for marine technologies that is proposed by the Scottish government.