In the current context of environmental degradation and depletion of fossil fuels, marine energy has emerged as an alternative to traditional energy resources. However, being in an initial step of development, extracting energy from the ocean is often regarded as difficult and uneconomical. The existing models for assessing marine energy costs are often oversimplified, leading to uncertainties that may hold investors back and slow down the market penetration of this renewable. Therefore, an accurate prediction of marine energy costs is fundamental to drawing conclusions about its competitiveness. Among the different possibilities of marine energy, this paper focuses on the economic analysis of tidal, wave, and offshore wind energy. The individual costs involved in the construction of these offshore energy parks and operation and maintenance tasks during their lifetime are determined. With this information, the levelized cost (€/MW h) is calculated for offshore wind, wave and tidal energy (€165/MW h, €225/MW h and €190/MW h, respectively). It is found that these renewables have a higher energy cost than traditional energy sources; however, considering factors such as the learning rate or externalities enhances their competitiveness. In the second part of this paper, combined energy systems, such as hybrid converters, are presented as a future solution to boost the development of marine energies. The synergies between these renewables are outlined, as well as the cost savings that can be achieved through diversified energy systems.