Spiny little brutes that can often be found using bare feet at low tide in the waters of the southern UK. Weaver fish are impossibly difficult to spot, as they dig themselves into the sand, leaving only their poisonous spines protruding.
The surfers' cure for a weaver fish sting is to pee into a wetsuit boot and place the affected foot into the warm, soothing liquid. This, although sounding like an old wives' tale, is actually quite good advice. The weaver fish, like most poisonous sea creatures, has temperature-specific poison and warm, or preferably hot, liquid prevents it from working.
An alternative way to cure the pain would be to hop to a local café and beg to use one of their weaver fish buckets, which are specially kept in Cornwall and other surfing areas. Fill the bucket with very hot water and keep the foot in the water for as long as you can, until the pain subsides.