The theory is that since his body was compromised by alcohol & drugs in the past, it was very easy for his blood pressure to drop while in it & he either drowned or just died due to the drop in pressure.
Reportedly he had prescription Chronic Obstructive Pulmonary Disease (COPD) drugs in his house. COPD drugs are prescribed for people with chronic breathing diseases, such as emphysema or chronic bronchitis (no recreational use). He was a life long chain smoker so it would be surprising if he did not have a lung disease.