The link to perpetual motion in the inital posting has unfortunately a bug. The correct link is this: https://en.wikipedia.org/wiki/Perpetual_motion But when I talk of perpetual motion, then for me a machine running 1000 years is just very fine, but not so for the theoretical idiots out there in the sciences etc. They mean it has to run forever, otherwise it's worth nothing... IMO that's sicko thinking!... They should know that it never can be fullfilled b/c the universe had a beginning and has an end, so nothing can last any longer than the universe itself. But as said just a fraction of the current age of the universe, say just 1%, is more than enough, as it would mean more than 138 millions of years the machine runs...
One of the main issues with your proposal for tidal energy is that the whole contraption might get clogged up with seaweed or dead fish or other marine creatures. I've always suspected we could extract energy from the activity of ants or termites, I just haven't built a machine to do it yet. But watch this space.
I even feel guilt when I eat eggs or honey, or drink milk, b/c we just steal these food products from the poor animals...
When i was about 10 years old i came up with this perpetual motion go kart design: Glad to know I am not the only one: