In the UK, the types of fish that can sting are weever fish, stingray, sea urchins and jellyfish. The weever fish is a small sandy coloured fish that usually lies buried in the sand on the seabed. It has spines on its back that contain venom so if you accidentally step on it you will be stung. Sea urchins are found on rocks and in seaweed and sting in the same way. The Portuguese man-of-war is a large poisonous jellyfish-like creature found in the UK. It has long tentacles and floats along the surface of the water. These creatures are generally non-aggressive except when they are disturbed.
If you are stung by a weever fish or stingray you should remove the spines and wash the affected parts of your body in very hot water for about 30 minutes to kill the venom. Use the hottest water you can possibly tolerate without scalding yourself.
If a Portuguese man-of-war or a sea urchin stings you, you should bathe the wound in vinegar, and remove any tentacles with sticky tape. If you cannot remove the stings yourself you should see a doctor for help.
As an immediate measure you could seek advice from a lifeguard.