In many regions strong tidal flow occurs only in areas with restricted water depths, typically within estuaries or rias. Although in some of these areas the depth constraint may preclude the exploitation of this resource, in others it is exploitable – and substantial. The objective of this work is twofold: (i) to develop a tool, the Tidal Stream Exploitability (TSE) index, to facilitate the selection of tidal power sites in depth-limited zones, and (ii) to demonstrate it with a case study. The TSE index combines the flow and water depth information so that the areas with potential as prospective tidal power sites present large TSE values. On the contrary, areas of lesser interest – of weak flow, too shallow, or both – have small TSE values. In the case study (Ria de Ortigueira, a large estuary in NW Spain) a numerical model of the hydrodynamics is implemented. Once validated based on field data, the model is used to compute the flow velocity and power density in the estuary at different moments of the tide. Two areas present high values of power density. One is unsuitable for a tidal stream power plant due to its shallowness; the other, which does have sufficient water depth, clearly stands out in the TSE map. Thus, the TSE index is shown to facilitate the selection of tidal stream sites in depth-limited regions.