Watch out for Weevers!
The Lesser Weever(fish), Echiichthys vipera, is a small, sandy coloured, fish that lives around the shores of Great Britain. It is only about 15 cm long but it should definitely be avoided due to the poisonous spines in its black dorsal fins It often hides half buried under the sand at low tide.
If you are unfortunate enough to step on a weever fish and allow it to discharge its venom through its spines and into your foot you are likely to experience intense pain! The pain will be at its worst for the first two hours and your foot may become red and swollen and you may be left with some irritation and pain for up to two weeks after the sting. If a spine has broken off in your foot you might need to visit a doctor to have it removed.
Death is extremely rare. The only recorded death in the UK occurred in 1927, when a fisherman off Dungeness suffered multiple stings. There is some suspicion that the victim may have died of other medical causes exacerbated by the stings. 
Treatment for Weever Fish stings
The affected limb or wound should be immersed in water as hot as can be tolerated comfortably for 15 minutes but, as a rule of thumb: the longer the delay (before heat is applied) the longer the treatment should be continued. Some cafes in Cornwall and other surfing areas keep a bucket for this purpose.
This deactivates the venom, which is a protein and therefore is denatured at high temperatures, and within a few minutes pain should subside permanently.
If this isn't possible, cover the wound with hot flannels which should be changed repeatedly.
Simple painkillers such as paracetamol will also help to relieve the pain, antihistamines may also be helpful.
 British Marine Life Study Society
Image: Tammy Horton (Published on the MarLIN website)