Interest in the power potential of tidal streams is growing worldwide. While the latest assessment for Canadian coastlines estimates a resource of approximately 42 GW, these results are based on the average kinetic energy flux through the channel. It has been shown, however, that this method cannot be used to obtain the maximum extractable power for electricity generation. This work presents an updated theory for the extractable power from a tidal stream in a channel linking a bay to the open ocean. The maximum average extractable power from a channel linking a bay to the open ocean may be estimated, within approximately 15 per cent, as 0.22ρ gaQ0, where a is the amplitude of the dominant tidal constituent in the open ocean and Q0 is the maximum volumetric flowrate in the undisturbed state.