A teenage boy was injured while "tombstoning", or jumping, 60ft (18m) from the top of a yacht club building into the shallow sea below.
The 15-year-old landed in just 3ft (0.9m) of water between the Hoe and the Barbican in Plymouth.
Ambulance crews called to the scene alerted the coastguard and the teenager was rescued by lifeboat.
He suffered a broken leg and possible spinal injuries in the incident on Saturday afternoon.
"What looked great on Youtube might not look so cool from... a wheelchair"
Coastguards said the Plymouth teenager and a "crowd of other youngsters" had been "tombstoning" from the roof of the Plymouth Royal Corinthian Yacht Club.
Zoe Boyne from Brixham Coastguard said a local should have had a better understanding of the unpredictability of the sea:
"What was 'safe' a few hours earlier may be the depth of a damp flannel now.
"What looked great on Youtube might not look so cool from the perspective of a wheelchair."
Ambulance crews took the youth to nearby Derriford Hospital.
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