Flip the oscillators, the sea is the tree branch, and the swing is the moon. The tree branch is moved slightly each swing, and if I am to extract energy from t

Flip the oscillators, the sea is the tree branch, and the swing is the moon. The tree branch is moved slightly each swing, and if I am to extract energy from the branch by forcing it to do work, I will damp the motion of the swing and it will eventually stop. You're right in that it's not renewable, the energy is being extracted from the kinetic energy of the moon, and it will slow the moon to some minute degree.