May Boatwright commits suicide by drowning. Though it comes as something of a shock, May's death is hardly a surprise. On the basis of the available evidence, it would seem that she was suffering from depression. Although May's family were always very sensitive to her condition, and always tried to create the best possible environment for her, there would always be a limit as to how much she could cope with the world and all its ups and downs. Sadly for May, the arrest of Zach for a crime he didn't commit is just too much for her, and pushes her over the edge.
There's something deeply symbolic in the manner of May's death. She weighs herself down with a heavy rock just as the world itself always weighed heavily upon her shoulders. There's also something ironic about someone with the name of Boatwright committing suicide by drowning. A boatwright is someone who builds boats, which are of course used to navigate rivers, along with other bodies of water. Despite being a Boatwright, May could never successfully navigate the choppy, turbulent waters of life.