Here are the dangerous sea animals, in no particular order

Beaked Sea Snake: Their highly toxic venom in nausea, vomiting, dizziness, collapse, and even convulsions. Dubois' Sea Snake, These snakes occur around Australia, Papua New Guinea, and New Caledonia. Their venom is highly toxic and may further cause respiratory failure.

Stonefish: Spines containing venom that when ever envenomaed it could cause high intensity pain and edema combined with respiratory distress.

Stonefish

Great White Sharks: Not normally aggressive to humans, they could deliver serious injuries or fatalities with phenomenally powerful jaws and teeth. Saltwater Crocodile: Where brackish waters are associated with the coastline, the powerful bite could mean serious injury or death.

Great white Shark

Blue-Ringed Octopus: Extremely venomous, their poisonous bites cause paralysis of the respiratory system; followed by respiratory failure or death in most cases.

Urchin Flower: Their spines contain venom that creates sharp pain, swellings, and respiratory troubles.

Urchin flower

Striped Pyjama Squid: The toxin contained in it is extremely toxic; even if it causes paralysis, respiratory failure, and eventually death.

Striped pyjamas squid

Lionfish: Even though they are mostly harmless, their sting inflicts great pain and swellings. Make sure to be very careful while swimming or diving if you happen to know any such animal ever existed in such a place.