I didn't quite understand how's that arsenic (or some word like that) work. I thought it would be enough only to open the door and it will explode, but how could she get away then? Brilliant film, though. I love it.
Teddy Lewis, the arsonist (not arsenic), told Ned that the explosive device he made for Matty had "a little delay"... probably just long enough for Matty to open the boathouse door, then dive into the water without getting hurt.