Interesting thread. I have assumed that the cabin boy also carried a wire saw, drilled two holes above the waterline and one below, in a sort of inverted triangle two or three feet across, and threaded the wire saw through each pair of holes and cut through. When the last cut was mostly done, water pressure would push the triangle of wood in, and a gaping hole would appear, sufficient to sink even a large galleon in little time if the hole wasn't found and plugged almost immediately. Of course, the cabin boy would have to be ready to swim away as soon as the wood gave way, or he'd be sucked into the sinking ship along with the rest of the water.