Every swimming beach you visit on any of the main islands will alert you to the potential dangers of marine stingers, the likes of box jellyfish (ハブくらげ; habu kurage) and venomous snakes – also called habu – the latter, being a kind of viper unique to the Ryūkyū islands whose bite can be deadly. But what are the risks? The answer to that question, of course, depends on who you talk to.
Between them the Southwest Islands have hundreds of kilometres of mostly spectacular coastline and visitors are told they should only swim in the netted areas of designated 'swimming' beaches, but that precludes many of the 'secret' beaches that you might come across on your own intrepid explorations. The venom of the box jellyfish is capable of causing paralysis and cardiac arrest in humans and, at least, an incredibly painful sting.
Deaths caused by both box jellyfish stings and habu snakebites are uncommon, but the threat is real and shouldn't be treated lightly. Wherever possible, do swim in netted areas, and if you just must swim in a non-patrolled zone, be sure to check with locals if jellyfish are about. Camp cautiously and wear covered shoes if you plan to be traipsing through long grass and if you find yourself feeling like you've been bitten or stung, seek help immediately.