complete disregard for the Thai rules and regulations before venturing into kayaking & water sports as noted on the sign board.... and no vests FGS.
I wouldn't put too much effort into looking for him in this regard, why put others at risk.
No wonder park authorities despise farang who think it's fine to ignore the rules.
But also, how does one drown or go missing just by falling off a kayak ? unless he went over a waterfall and hit a rock or some accident similar.