This idea has been tossed around up in the Maritime Provinces for years - the Bay of Fundy has colossal tides, some of the biggest in the world, and it's occurred to more than one person over the years that you could get some serious work out of that if you built the right equipment. There's one generating station up there now, but as you might expect, the environmental lobby protests it regularly because it's a dam and Dams Are Bad. (Someone's working on another system that just sort of sits in the water and uses the flow as the tide goes in and out, but there are questions about whether that'll ever be efficient enough to bother with. Be pretty cool if it worked, but I'm sure the greens will find something about it to picket if it does. The turbines could suck in a fish or something.)