Go through these in order and stop at the first one that applies. If you are reading this for someone else, answer for them.
1
Is any of this happening now?
Call 911
Call emergency services now if there is:
Trouble breathing, collapse, confusion, paralysis, or a seizure
Chest pain, severe weakness, or rapidly worsening symptoms
Swelling of the tongue or throat, wheezing, or widespread hives
Bleeding that does not stop with firm direct pressure
A large or deep bite, major tissue loss, or an embedded barb
A suspected box jellyfish, cone snail, blue-ringed octopus, sea snake, stonefish, or other highly venomous exposure
Follow local emergency instructions immediately.
Other emergency signs, for anyone
Someone who is struggling to breathe, gasping, choking, or cannot get words out
Someone who has chest pain, tightness or pressure lasting more than a few minutes
Someone who has sudden weakness or drooping on one side of the face or body, slurred speech, or sudden confusion
Someone who is unconscious, or cannot be woken
Someone who is bleeding heavily and it will not stop with firm pressure
Someone who has had a seizure that will not stop, or does not wake up afterwards
Someone who has lips or skin that look pale, blue, grey or blotchy
Someone who you believe has taken an overdose or swallowed something poisonous
2
Is any of this happening?
Be seen today
Seek prompt medical assessment for a sting or wound involving the eye, mouth, neck, chest, abdomen, genitals, a joint, or a large area of skin.
The same applies if a spine, tooth, or tentacle cannot be safely removed.
Get urgent help for worsening pain, rapidly spreading redness, fever, pus, skin blisters, or darkened skin.
Do not wait if you have liver disease, diabetes, poor circulation, or reduced immunity and develop symptoms after a seawater-exposed wound.
And for anyone, someone who:
has a wound that is hot, spreading red, or leaking pus
3
Does any of this apply?
Book an appointment
Routine follow-up is appropriate when a wound is healing but tenderness, itching, a lump, numbness, or limited movement continues.
The article also says
A small puncture from a cone snail or blue-ringed octopus can be dangerous even if it is not very painful.
Every sign above is quoted from the sofidex article on Marine animal stings or bites. If you are not sure, err upward — it is always reasonable to be checked.