The beak of a blue-ringed octopus is built for small shells. On a person it can leave a mark that is easy to overlook, and a number of medical accounts describe the bite as painless or nearly so. That is the detail that makes the animal more dangerous in practice than a creature with an obvious wound. Pain is a useful alarm. An alarm that does not ring leaves a person standing in the shallows, annoyed or unaware, while the clock on the toxin has already started.
What follows, if the dose is a serious one, is numbness, weakness, and then paralysis. Breathing becomes the emergency. Nausea and failing circulation are reported. Vision can be affected. The progression can be fast, on the order of minutes, which is why the correct next move is emergency help and not a period of waiting to see if the odd feeling passes. Children are at greater risk for the plain reason of smaller bodies.
There is no antivenom. Treatment is support, above all support of breathing, until the body has broken down and cleared the tetrodotoxin. People who are kept breathing through the paralysis routinely survive it, and those who are alive after the first day are generally on the way to a full recovery. The toxin does not, in the usual account, leave a person ruined if respiration was maintained. It leaves them dead if it was not.
This is not a treatment protocol, and it is not a substitute for local emergency instructions or a clinician. It is the shape of the risk, stated so the pretty animal is not misunderstood. If there is any chance a person has been bitten, the errand is an ambulance, and the useful bystander is the one who can keep an airway and breathing going until people with equipment arrive. Do not cut the wound, do not chase the octopus, and do not decide it was nothing because it did not hurt.