As my Australian friends told me:
"Everything in Australia bites - and it's poisonous."
An exageration to be sure, but actually not that far off!
As far as swimming in the ocean, witness the Blue-ringed octupus:
http://www.didyouknow.cd/animals/octopus.htm
It never ceases to amaze how people swallow the line that everything bites in Australia. Perhaps, they have swallowed too many Aussie tall stories.