Jellyfish is consists mostly water and they live in the ocean, I like to create a world of airborne jellyfish how can I explain the levitating mechanism and evolutionary advantage scientifically?
Large, gas filled jellyfish that float around have been hypothesised as a type of life form which could be found in the atmosphere of a gas giant such as Jupiter.
Respiration in jellyfish would not be a problem because they absorb oxygen through their thin skin. The problem with this is that without water, their skin may break easy needing some kind of mucus or protective boundary which allows the diffusion of oxygen but still keeps the jellyfish moist.
The problem now is the flying. If the jellyfish were filled with a lighter than air gas, then they would be able to fly, possibly controlling their ascent and descent by pulsating their body. To prevent the gas from leaking, an organ for holding it would have to be developed.
Jellyfish need salt for their nervous system to work properly, so these flying jellyfish will need to dive back into the ocean every so often to keep their salt levels high. During these dives is probably when they would feed. This limits the distance inland that the jellies can venture.
In the air, jellyfish have less predators, fending off birds with their stinging tentacles. This dramatically increases the survival rate, and also may allow the jellyfish to have access to new food sources such as small mammals and birds.
Most fish have a bladder they can fill with air to control their buoyancy, electric eels can produce an electric charge and water can be broken down to Oxygen and Hydrogen with electricity.
So you have a jelly fish that can produce an electric charge, originally for self defense. The man-o-war which apparently isn't a true jelly-fish has a large air bladder for flotation. By some happy accident our particular jelly fish starts producing hydrogen with its electric charge. maybe as a way to help it ascend faster, using the Oxygen in it's other processes.
Eventually it starts floating its main body a little above the water, trailing it's tentacles, and with only the tentacles in the water fewer of them die and are eaten. They grow longer tentacles to float higher up, periodically catching insects or birds...