How about this scenario.
" He was diving to excess, longer that normally advisable, to make the most of his holiday. He comes back to his hotel room tired and takes a rest before the evening's party activities. ( he was on holiday, so entitled to enoy himself with his friends.)
Many people sleep in boxer shorts, so nothing suspicious abut that.
To get in the party mood, he takes a handful of chemical simulants. These on top of excessive exercise diving, are too much for his heart, so he has a cardiac arrest. It is a well known medical fact that death often follows a heart failure.
The musty smell in his room can easily be explained by wet clothes/towels, bad drains, and no windows open.
Poisoning is for the conspiracy theorists, could anyone forcibly administer poison to him without a struggle?
AN followers should be aware of the Thai tendency to run from or any trouble, so not surprising that locals take the easy way out.... "Nothing suspicious to see here, just heart failure"
I am sure that Local Plod remember all the trouble from the two murders a few years ago, so want this to go away as quickly as possible. Hotel owner similarly does not want any bad publicity, so quickly brush this away."
Is there any requirement for a thorough medical before taking PADI diving course?
Was there any reason why anyone would want him dead? It is not usual to murder someone for no reason
I do not believe that any local thug would have the intelligence or means to administer some difficult to trae poison. More likely to beat him to death.