Jack is left sputtering from the sudden splash. Couldn't be a wave on such calm waters. Sharks are seeming more likely. Or worse things. He wipes the sea water from his face and plucks the kelp off of his head and shoulders as he watches the bottle bob along. He tilts his head at it. "Tha's not right..."
But then the thing disappears. It was all far too sudden to be the natural way it would take on water. Jack goes onto his knees and holds onto the side of the boat, peering down into the water to see if he can glean where that bottle got off to.