
I converted this to a movie clip, and then duplicated it many times. Once I had a large row of them across the width of the canvas I resized them so there would be greater variation between the size of the bubbles, and I also changed the transparency of them to create an impression of depth. This is what I ended up with:

I placed them on a keyframe at the start, then inserted another keyframe where I wanted it to end, and moved them all to the top. I then distrubted them to a layer each and created a motion tween. By doing this they all moved to the top, but now they were all on their own layer I could manipulate them individually. I moved both keyframes to different positions on the timeline to create a variation in speed, and then moved some of the bubbles around on the final keyframe so some of them would move a bit more instead of just going straight up.