It may still be too soon to tell, but most of the foreshadowing points to Bonnie being the one who drowns. It's clear that she's holding onto some secrets about her life before moving to Monterey, and it doesn't help that her own mother is the one who has the visions of her drowning body. Bonnie could be so crushed by the weight of causing Perry's death that she ultimately takes her own life — or, perhaps Mary Louise (currently on the war path to find out what really happened to her son) finds out that Bonnie is his killer, and pushes her into a swimming pool?
There's also the possibility that nobody drowns in the present day, and instead we find out that someone (along with Max Wright) suffered a near-drowning incident in the past. And with the new revelation that Perry's twin brother Raymond died in "an accident" at 5 years old, there's also a chance that the abusive husband and father may have taken his brother's life — perhaps by drowning him.
