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?
I agree. If she was murdered, the killer was on the plane. If not the pilot (who had to be part of the plan to ditch the plane) then the passenger list needs to be checked very, very carefully.