When I visited Koh Naka Noi, the smaller one of the two Naka islands just off the North East of Phuket. I was chatting to the boat captain, and didn't notice the rocks.
I've seen this described before, the boat's engine is turned off, the waves are still and the rocks drift across, as if floating.
Yes they are rocks, and not debris.
It's an optical illusion. The waves are actually moving, the current is the same near the rocks and under the boat, so the boat moves and yet appears stationary against the waves.
The rocks are still, the boat and waves drift in harmony and that causes the effect. It's a pity to spoil the magic, but keep an eye out for it, perhaps you can impress your kids with it!