Prince's Sporting Club in Surrey has pleaded guilty at Southwark Crown Court to corporate manslaughter after 11-year-old Mari-Simon Cronje died when she was hit by a boat towing the inflatable she was riding.
Company director Frederick Glen Walker was cleared of any offences after the Crown Prosecution Service dropped charges against him.
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Paramedics attended a scene in Willesden, north-west London, on Monday afternoon where they found the teenager with stab wounds.
At least five people have suffered minor injuries but the cause of the blast is still unknown.
A songwriter from Harlesden who wrote a track for Beyonce's Lemonade album has been nominated for a Grammy Award.