Would this be a practical way to convert tidal energy into electrical energy? Imagine a structure that is held above the water by pylons that are grounded on the ocean floor. In between these pylons is a pontoon that, when tides are rising holds and lifts a heavy weight. When the tide is falling the weight is supported by the structure, which can gradually let the weight down, and in doing so create electricity. Would this be a practical or even viable way to create electricity? Why or why not?
 A: Yes except it is normally expensive to sink piles into the ocean floor. 
It would only work over a narrow range of tidal height and in shallow water and risks being damaged in a storm.
This system is normally used to with waves rather than tides. Tides would only lift your weight twice a day, while waves could lift it many times/minute. Although tides can be larger.
See https://en.wikipedia.org/wiki/Wave_power 
A: Good thought. Same concept though, I would say, a one way water tank may be more practical, where water tank fills when the tide is high, and it empties to generate power. Or some kind of water locks can do the trick too. It may be more economical, if there are suitable geographical locations to hold huge amount of high water with minimal lock mechanism needed and then use it for power generation. Actually, this way, you can also generate some power even when the water goes through the lock during high tide. So, both ways.
A: The key to making optimal use of tidal lifting capability, without undue impact upon aquatic life is by using the vast air-displacing capacity that rising and falling tides can create in an inverted dome. The dome itself would support a pier upon which normal civic life and economic activity would continue undisturbed. When the tide rose, air would be compressed and discharged through turbines to generate electricity. When the tide fell, atmospheric pressure would cause air to rush back into the dome through turbines, generating electricity. The system would have to be integrated with a larger electricity supply system to be of optimal benefit, but with that accomplished, the approach would have long term practicality and be carbon neutral, courtesy of the sun and the moon. The only problem with this idea is that doesn't have huge potential to make anyone super rich, so it will probably never be actualized.
