Irikanji are no fun - I reckon hundreds have 'drowned/cardiac arrest' over the decades, but really died from being stung by irikanji (all over the top end, even found now down to Fraser Island in Qld.) They are so tiny, they don't leave the big welt marks of Blue Bottles & other stingers ..... yet they reckon only 2 have died! How would they know! They weren't even identified as a threat until the 50s & not made public to the actual public til probably the 70s or later!
https://en.wikipedia.org/wiki/Irukandji_syndrome