Marine generating devices are still in development, so it is too early to judge their long-term reliability. All types of marine electrical generation are to some extent weather-dependent and thus intermittent.
Some types of marine power are more reliable than others. For example, electricity generated from waves depends on wind, because the strength of waves is driven by the wind. Tidal power is more reliable because tides have a daily cycle: their time and force at different points along the UK coast can be predicted.
Peak electrical output from tidal systems occurs at the peak movement of the tide, which may or may not coincide with peak demand. We can alter this by using lagoons, which hold the water until it can be run past turbines at times of peak demand. But lagoons require more land to be devoted to tidal energy schemes and the number of sites around the UK is limited.
Improving the reliability of marine energy as a source of electrical power is challenging because of its dependence on weather conditions.
Despite relying on the activity of the wind, wave power is potentially more predictable than wind power. And its capacity to generate electricity increases during the winter, when electricity demand is at its highest. Tidal and current stream energies are more predictable and consistent, but may also be intermittent.
Marine technology is in its early stages of development, but promising trials point to its potential as a contributor to the future UK energy mix after 2020.