>>>the south-shore of Oahu gets a bunch of box jelly fish...
>
>Didn't know that. There are multiple variants so from your description, I wonder whether you get the awful version seen in Australia that can blind or kill. Its transparent body can be the size of a basketball with each corner sporting a trailing tendril 2 or 3 yards long, which is what delivers the injury. You can't expect to see it or avoid it- in fact, for years people weren't sure what was causing sudden agony and death in Australian waters.
>
>Meanwhile the ghastly-looking Portugese Man-O-War causes 10,000 stings on Aussie beaches in summer, but rarely kills.

Then there are the ones you don't see: https://en.wikipedia.org/wiki/Irukandji_syndrome . Check the size of the culprit.

The Wiki article used to include some descriptions from victims. Those can still be found at http://godheadv.blogspot.ca/2010/12/limits-of-pain-irukandji-syndrome.html