[eraserj]

But again, slowing tidal currents must be drawing energy from somewhere until there is none left.

[Xylakant]

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.

[tshaddox]

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).