Emergency services have reissued their drowning-prevention advice after the young man was shown balancing on a post in the water before jumping off into the lake.
The Facebook video showed his friends playing on a boat nearby, watching on as he took the leap.
The Brigade's head of community safety, Chris O'Connor, said: "It's incredibly dangerous to jump into water like this because it's impossible to see hidden obstacles under the surface.
"It's very easy to slip and get into difficulties."
Brigade urges people to take care after Serpentine 2014 death
In 2014, 26-year-old Alexander Mitchell died from drowning after jumping into the same lake following a large consumption of alcohol.
The Brigade was called after people said he had not resurfaced and they later found his body.
Mr O'Connor added: "It's tempting to jump into water like canals, lakes and rivers to cool down during the warmer weather, but it's just not worth the risk.
"Sadly people drown each year and we'd hate for someone else to become another statistic for the sake of having a laugh or showing off to their mates."
National figures show 338 people died from accidental drowning across the UK last year, with a quarter of victims found to have alcohol in their system.
What to do if you think someone is drowning
The Brigade's water safety advice is as follows:
If someone falls into deep water the first thing is to call for help straightaway.
Call 999 and ask for the fire service and ambulance. When you have made this call, shout for help from anyone who might be close by.
Never enter the water to try and save someone. This can add to the problem even if you are a strong swimmer.
If there's no lifesaving equipment, look around for something to reach out to them such as a scarf or a long stick.
Lie on the ground so your body is safely on the edge to avoid being pulled in.
If you do manage to get them out of the water, always seek medical attention – if water has entered the lungs then it can cause death up to 48 hours after the incident.