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.
When Alisdair is confronting Ada about her love for George, when they are inside the house, the light in the windows is clearly from bright sun. Through the open door, no rain is seen. But when he drags her outside, the rain from the previous scene has returned and the day is very dark.
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.