I don't post often, I'm more of a lurker, but I wanted to address your theory.
I think you are definitely on to something here with this theory. I snipped the following from the Aspen Times:
Specifically, Kurtzman's report cited “marked pulmonary edema,” or fluid buildup in the lungs, and “acute intermittent porphyria,” a metabolic disorder that causes severe stomach pains. (BBM)
http://www.aspentimes.com/article/20120907/NEWS/120909901/1077&ParentProfile=1058
I've been looking into this possibility since you posted this theory and have found some interesting things concerning the use of oxygen restriction due to "waterboarding", but I don't really know where to go with it considering my lack of medical expertise. Anyhow, ingestion of water and contaminates causes both hypoxaemia (which from what I've been able to find can cause pulmonary edema) and gastrointestinal disturbances (paraphrased from a research article I found through google), which might explain the original cause of death being both pulmonary edema and acute intermittent porphyria?? Additionally, the experience of this can also cause cardiac arrhythmia and arrest from the fear and stress (also from the research article).
The article is
"Not waving, drowning". Asphyxia and torture: the myth of of simulated drowning and and other forms of torture by Jonathan Beynon, MD (if you want to take a look).
If anyone can weigh in on this with a medical background, that would be awesome!