What made the water of the Jedi Temple pool ripple?

Question

In The Last Jedi, Luke Skywalker went to the ledge, outside of the First Jedi Temple, laid his hands on a large stone, and started meditating. Soon, the water in the pool inside, with the mosaic of the Prime Jedi, began to ripple. What was the cause of this?

A friend of mine suggested it was because of Luke's reconnection to the Force. But what did the water pool have to do with it? Was it some kind of a tool for connecting and disconnecting oneself with the Force?

Another friend thought the water somehow resonated of Rey's entering in the Mirror cave of the Dark Side. It happened roughly the same time in the movie, and maybe the cave was right underneath the pool.

Could someone please explain what made the water in the pool in the Jedi Temple on Ahch-To ripple?

Answer 1

I think you've hit the nail on the head. Luke was, at the very moment that the pool of the Jedi began to ripple, reconnecting with the Living Force. The 'raw energy' of that connection, in conjunction with the Temple itself being charged with the Force energy of thousands of Jedi acolytes is presumably what made the water move.

Luke sat, his legs protesting as he forced them into position. He put his hands on the rocky ledge, where so many Jedi had meditated over the eons, and closed his eyes. Breathe. Just breathe.
The wind filled his ears—the island’s constant companion. It was a whisper now, the low conversation of autumn breezes instead of the whine of winter or the howl of a summer storm. He could hear the night birds calling as they rode high above him, and the metronome calls of insects from the grasses.
Behind him, in the ancient temple, the still surface of the water in the ancient font began to ripple and dance.

_{^{Star Wars: The Last Jedi: Expanded Edition}}

For the record, Rey hadn't quite entered the Cave of Mirrors yet (since Luke was able to use the Force to watch her as she fell), so it's unlikely that that was what was causing the water in the font to dance.