There is no truth to that urban legend, no. Most urban legends are fabrications, and some are based on actual occurrences. However, the dead scuba diver found in a tree is false, and there have never been any recorded incidents similar to this. Most of the helibuckets -- which are the devices used by helicopters and airplanes to collect water in the event of a forest fire -- have opening far too small for any human being to fit through. And with all the scuba gear, it would be near impossible to fit through the helibuckets' openings.
The largest helibucket can carry ten-thousand gallons of water, but the opening is a ring with a one-foot circumference. Bombarider water bombers have two intakes, but the intakes are four inches by ten inches, which, again, is too small for a diver with scuba gear to fit through. In addition, the intakes are covered by grilles, so it would be nearly impossible for a human to get in. Helitankers have intakes that suck water through what is called a "donkey dick", and the opening is only a couple of inches in diameter. Thus, it would be nearly impossible for a scuba diver -- equipped with gear and all -- to get into one of the intakes. So, the legend is false.
As far as the legend goes, like most urban legends, it plays with commonalities in society and peoples' phobias. The "commonality", if you will, in society is many people enjoy discussing peculiar and comical ways of dying. It's like the Darwin Awards. The Darwin Awards honors humurous and comical ways of dying. People like reading the Darwin Awards, and thus people enjoy comical ways of dying. There is a slight hint of humor in this urban legend, as the death is so farfetched and almost implausible that it's hilarious. Thus, someone may have created this tale in order to play along with society's commonality in the enjoyment of comical deaths.
Or, if could play with numerous phobias that some people may share. Many people are afraid of dying in freak accidents, and this is a freak accidents. Some people are afraid of being caught in the wrong place at the wrong time, and this, too, plays with that fear. It may also play with the a fear of heights. In conclusion, someone may have created this legend to play with peoples' "illogical" fears of certain things.
Or, the legend could be originated from a similar, yet insimilar incident that occurred off the island of Corsica in 1998. A scuba diver was caught in edies -- mini water tornadoes -- caused by a Canadair airplane, and was thrown against a landing stage, which is a dock or platform where passengers and cargo can be loaded and unloaded from water planes and boats. The diver suffered a bruised leg. Thus, someone may have used this occurrence to create the tale of the scuba diver in the tree. This might not be the exact origin of the legend, as this folkloric tale has been told since 1987.
Despite the eleven-year gap between the estimated origin of the urban legend and the occurrence in Corsica in 1998, the incident of the Corsican scuba diver may have causes the urban legend to continue to be told throughout the years. The fact that something involving a scuba diver, an aircraft and water may have causes people to play with that occurrence, and recreate the belief that helibuckets were able to "inhale" humans.
What may have also caused the legend to survive throughout the years might be peoples' fascinations with the puzzlement of solving the mystery behind the legend. The legend can be turned into a riddle -- "A scuba diver is found in a charred tree — how did it get there?" When someone solves the riddle of the scuba diver, they tell someone else, and once someone else receives the solution to the riddle, they tell someone else, and the teller of the riddle can have an "I'm smarter than you are!!" moment. And the riddle is spread, and when incidents like the 1998 Corsica incident
So, in conclusion, the legend plays on society's shared phobias and entertainements, and this may be what causes the birth of the folklore. What causes it to last so long can be attributed to the mystery of it. When a riddle is told by someone to someone, and when the receiver of the riddle finds the solution, the person is likely to tell someone else the riddle. It's the same for the tale of the scuba diver found in a charred tree. Also, when certain incidents, like when the scuba diver was pushed into a landing stage, occur after the birth of a legend, it causes people to spread it more, and maybe play with the actuality of the incident to fictionalize the incident.