That is, jumping off a pier or a harbor wall into the sea. It seems to be the coastal British press that has to deal with this, chiefly, and that's where the term is residing. It's not any sort of a new craze or anything, not by a longshot. The Royal Society for the Prevention of Accidents has a whole page and everything on it, a signifier of how often they have to deal with this that they had to make a page for it.
There are two major problems here: first, you probably don't know what's under that water or how deep it is. The lifeguard who jumps in after you knows, but you don't. Second, you don't know if there's a riptide waiting to drag you out to sea... which is why the lifeguard will be jumping in after you. And this doesn't cover the potential shock of cold water, or your ability to swim away even if you don't get pulled out to sea (two more reasons for said lifeguard).
Now, this is not to say it isn't an inherently bad thing to jump. Recently in Ocean Beach, CA, about 600 kids jointly jumped off a 30-foot pier. But there's a reason they did it: they were junior lifeguards in a four-week water safety training program, they were under the watchful eye of real lifeguards that, again, knew exactly what the conditions were at water level, and it was done with the express purpose of training them not to be afraid of the water. They paid a $75 donation to the Junior Lifeguard Foundation to get to do it. Their parents jumped too.
I hope I should not have to tell you not to try it.