Blue ringed occie is about the scariest thing I can think of in the world.
You won't see it
You probably won't feel the bite
The muscle paralysis is total but as your heart and breathing stop, and you sink to the bottom, you'd be aware what is happening.
Many years ago there was a guy who was bitten, and as his mates drove him 1hr to hospital, doing CPR all the way, he could hear a person saying "give up he's dead"
I'll bet he doesn't talk to that 'mate' anymore but thinks the other 2 or 3 are just tops