Isn’t tidal energy just harnessing some of the existing gravitational potential energy of the Moon-Earth system, which is roughly similar to any system that harnesses energy from a massive object falling due to gravity?
But again, slowing tidal currents must be drawing energy from somewhere until there is none left.
It basically draws energy from the moon orbiting earth. It could theoretically alter that orbit, but given that the moon/earth system has a lot of mass, the timescale for that to happen is extraordinarily large.
That’s exactly what it’s doing. It’s slowing down Earth’s rotation and speeding up the Moon’s orbit (i.e. moving the Moon further from the Earth).