Here in the far southwest of the continent we've never had to worry about seasnakes, though as you say we are taught from age ~8? about stonefish, various jellies, sharks, blue-ringed octopus, conefish. Even normal octopus have a venomous bite, they just run away instead... we're also taught that any scratch contracted from coral is likely to get infected since marine-species bacteria is more aggressive or something... any diver will tell you that is false though.

I've had 3 Blue Ring Octopus sightings that I recall personally. The largest one I've ever seen was the size of a grapefruit; I had never heard of one so big before, but the tentacles were short and the rings were vibrant and it moved in my direction and I was veritably running on water in the opposite direction when I realised what I was looking at.