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Tidal power offers predictable astronomical timing, while wave power can exploit an energetic marine resource. Neither is constant: tidal streams have slack-water periods, and wave energy varies with weather and season. Diversity can help a renewable portfolio, but resource size does not by itself establish economical extractable output.

For scale, a mean marine output of $5\mathrm{GW}$ would produce $43.8\mathrm{TWh}$ per year, about $15\%$ of an assumed $300\mathrm{TWh}$ annual electricity demand. Achieving that mean at capacity factor $0.30$ would require about $17\mathrm{GW}$ installed. This is an illustrative deployment requirement, not a measured UK resource estimate or a 2012 forecast. Tidal-stream kinetic power density scales as $\rho v^3/2$, making energetic sites valuable, but channel-wide extraction also changes the flow, so isolated-rotor power densities cannot simply be multiplied by sea area.

Harsh marine conditions, access for maintenance, survivability in storms, cable connections, navigation and ecological effects constrain deployment. Barrages additionally change estuaries and sediment transport. Research and demonstration may be worthwhile for energy diversity and industrial capability, but “important to investigate” is weaker than “indispensable at any cost”. Marine investment should be compared with efficiency and other low-carbon options using delivered energy, system value and environmental impacts.

Useful power extracted from surface-water waves. Resource variability, extreme-storm survivability, offshore access, moorings and electrical connections affect practical performance. Its potential and costs are distinct from those of tidal power, even when both use marine locations.