There are a large number of venomous marine creatures from which you can receive a nasty sting. Most of these will just be a wound, but some have a sting that can kill a human. The first group that can sting are the jellyfish, which includes hydroids, anemones, and fire coral; these creatures pack a major sting and can result in a minor irritation to full blown blisters, rash, nausea, vomiting, shortness of breath, muscle spasms, and even low blood pressure. For most stings the wound should be flushed with water to remove any tentacles and then should be immersed in hot water for 20-30 minutes. Any remaining tentacles should be removed with tweezers and the area should be shaved. Then a hydrocortisone cream can be applied. Warm packs can be used for pain management. If the sting is suspected to be from a box jellyfish, then constantly flush the wound with vinegar (5% acetic acid) and keep the person still until medical help arrives.
Second, Lionfish, scorpionfish, and stonefish have dorsal, anal, and pelvic spines that can inject venom into a human through puncture wounds and usually cause redness, swelling, itching, and sometimes blisters. Stonefish stings can occasionally cause death. To treat these types of stings soak the wound in water as hot as you can stand it. This is most effective for lionfish stings, but might also be helpful for stonefish and scorpionfish stings. Then, use a topical antiseptic cream and change the dressing regularly. Divers will need to see a physician if they are experiencing vomiting, shortness of breath, weakness, or are unconscious after being stung. Patients will also need to seek a doctor's care for scorpionfish stings as these can takes weeks or even months to heal.