When the boat leaves the island, Ada trails her hand in the water, which is still and calm. On long shots, it is foaming from the action of the oars, and the boat on the water.
Pianos of the period portrayed in the film were made almost entirely of wood, no metal framing at all, and the piano would therefore float, not sink.
Ada writes a note to George on a piano key, but earlier George had told her that he can't read.