A proper autopsy would have detected death by drowning due to water in the lungs which was either absent or the autopsy report was fudged!
I think that if an assassin was hassling her, he could instigate a panic heart attack without holding her head underwater long enough for her to fill her lungs with water ie "drowning." Or, an assassin could give her a puff in the face with a small aerosol spray. Epinephrine is usually injected, and it can cause a "heart attack."
Some "serial killer nurses" have used Epi to kill dozens of patients, and it will only show a heart attack happened, because the body naturally produces Epi. And Epi is only one of many ways to induce a heart attack and beat an autopsy. It might have even happened to Breitbart.
See above about an Epi aerosol.
I’m thinking butyl or amyl nitrate (AKA “poppers”) as a possible arrhythmia inducing agent (inhaled vapors) if such was used. Easily carried, easily disposed of, common enough in club scenes and other circles. Who’d find a little bottle in the ocean?