Why do stones ripple when they fall into the water? How is it similar to electromagnetic waves?

The water molecule was hit by a stone and gained the momentum of the stone. The movement of the water molecule changed from calm and uniform to fluctuating and variable speed. It is this change of molecular velocity (including the magnitude and direction of velocity) that leads to the fluctuation of water.

Electromagnetic waves are different, not the fluctuation of molecules, but the change of the velocity of electrons in an object. Fluctuations caused by sudden strength form electromagnetic waves. If the current is flat and constant, then no matter how strong the current is, it cannot emit electromagnetic waves. No matter how weak the current is, if the speed changes, it will inevitably produce electromagnetic waves. The key is this change.