In McAfee's case, it is clear that he committed suicide.
This was shown by the official investigation, which even states that he left suicide notes in the pocket of his pants - but also his wife claims that he talked to her that day and promised to hear from her in the evening. A second independent autopsy has been requested, but has not yet been approved, so there will remain doubt as to what actually happened - because JM may have known some things he needed to take with him to the grave.
However, the case of Mircea Popescu looks very suspicious. What are the chances that someone like him would venture alone in to a section of the beach which is known to be dangerous?
Given his character and way of life, I wouldn't be surprised if he ignored all warnings and security measures, because when you have a lot of money in your life and you can afford everything, you start looking for excitement in some dangerous things - the biggest stake such people put on the gamble is their life.