The dangerous fish in question is the lesser Weever, which buries itself in the sand with just its eyes showing and its Black dorsal fin extended. When trodden on, it injects a venom into the foot of the unwary paddler. The sting gets more painful over time, paddlers believing they have merely scratched themselves on a sharp stone at first. The pain increases, as do the symptoms which include severe pain, itching, swelling, heat, redness, numbness, tingling, nausea, vomiting, joint aches, headaches, abdominal cramps, lightheadedness, increased urination, and tremors. Rare and severe symptoms include abnormal heart rhythms, weakness, shortness of breath, seizures, decreased blood pressure, gangrene, tissue degeneration, and unconsciousness.
Although extremely unpleasant, Weever stings are not generally dangerous and the pain will ease considerably within a few hours even if untreated. Complete recovery may take a week or more; in a few cases, victims have reported swelling and/or stiffness persisting for months after envenomation.
The only recorded death in the UK occurred in 1927, when a fisherman off Dungeness suffered multiple stings. The victim may have died of other medical causes exacerbated by the stings.
Information taken from Wikipedia.