I wonder whether a sealed environment could be used to generate power from tides. A sketch of an initial idea is below. The system has two tanks, one fixed then other free to move up and down with the tide (freshwater is less dense than sea water, so it should float). When the tide is in, the floating tank is up, so the freshwater flows to the fixed tank. When the tide is out, the floating tank is down, so the freshwater flows back from the fixed tank to the floating tank. Each time there is flow, energy can be extracted.
It is likely that some kind of active control of values would be necessary to optimise power generation, but the cost of implementing this should be minimal.