If it needs to be 2d,
I think you can start with a good texture (or a stack of animated textures) mapped on quads.
Searching around for a math formula to simulate some kind of jellyfish shapes could be a workaround too but more complex.
And yes you can use the flash renderer to play flash animation (could be the simplest).
If it needs to be 3d,
You can modelise it with a 3nd party modelling software and import the model as an x file or maybe go for dynamic meshes using some jellyfish formula (if that can be found not sure of that).
sorry,
very basic answer but @work and time lacking.
Maybe some good photoshop brushes could do the trick.
You can create one picture by jellyfish’s states (playing with photoshop tools to deform the initial texture in order to create the illusion it’s floating arround). That would be the tricky part.
You can use a spread of images into filetexture, accessing each pics via getslice and looping around getslice with an lfo.