Cornish coastguards have renewed their warning about 'tombstoning' following another accident on Thursday evening.
Coastguards say 'tombstoners' are not away of the water depth
An 18-year-old from Troon in Scotland is recovering in hospital after suffering head injuries at Piskey's Cove near Porthleven in Cornwall.
There is frustration that young people appear to be ignoring the dangers involved in jumping from cliffs, rocks and harbour walls into the sea.
Last month a man died after tombstoning at Ansteys Cove in Torquay.
About 40 young people were also cautioned by police and coastguards after jumping off rocks near Babbacombe in Devon.
In Cornwall, security guards are being paid by the district council to patrol Penzance harbour and the town's Jubilee Pool because of fears that teenagers are risking their lives.
John Rossiter from Falmouth coastguards said: "It really is a very dangerous game.
"People are just not sure of the depth of the water and they're toppling over and striking their heads and limbs on rocks.
"We ask people to please think about it. It's just far too dangerous."