1. These actually exist already...
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2. you could work out the math but im sure the power derived from such an endevor would be very little, and certainly not enough to cover the cost of construction. They do use small scale hydro electrics in small rivers in remote locations however, usually refered to as "micro hydroelectric" systems. Check this out:
http://www.eere.energy.gov/consumerinfo/factsheets/ab2.html
3. Windmills dont work too well in some locations, unless they are extreamly high and large. Certainly no where near cost effective in most locations unless you build an extreamly large one. I think this one is really neat, look at the size of it:
**broken link removed** , here is a good picture:
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4. They actually do this already. Its a really neat concept and can produce a great deal of power. check this out:
http://www.darvill.clara.net/altenerg/tidal.htm