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 to move.

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.