The Coastguard says it was alerted to reports that a woman wearing a life jacket was missing at sea at 1.30pm this afternoon.
The disappearance followed a collision between a yacht and a dredger, with one other person pulled from the water by a rescue boat before raising the alarm.
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Paramedics attended a scene in Willesden, north-west London, on Monday afternoon where they found the teenager with stab wounds.
The popular game relies on Google Maps, a problem in South Korea as those functions are limited by the government.
A Met Office warning covering much of England and south-east Wales is expected to remain in place until 11am.