Yes Octopuses can and do bite humans.
If you get close enough to an Octopus and the Octopus feels threatened then the Octopus can bite you and cause a pretty serious injury and bleeding.
So if you notice an octopus in the water do not go near it and get away from it and get out of the water.
In some cases some types of Octopuses have deadly venom that can be poisonous and deadly to humans when the human gets bit by the Octopus.
So if you do get bitten by an Octopus you need to seek medical attention.
Most times though the bite will just cause swelling and bleeding.
But if you're unsure of the type of Octopus that bit you then you need to see a doctor or go to urgent care or the emergency room.