Create the bubbling water animation (a whole thread/post in itself) in an MC
Nest water_mc in another MC. Create a mask to reveal only the 'loaded' portion of your show
Over 100 frames, tween the water_mc from below the mask (hidden) to fill the mask (fully visible). This should create the illusion the loader is filling up.
In the _root timeline place an action that parses _getBytesLoaded into a percentage. This number is then parsed into the current frame of the loader_mc. So at 50% load, the playhead of loader_mc will be at frame 50, thus appear to be 50% full. All the while water_mc should be bubbling away.
This is a very simple old school way of doing it, but it should get you started. I'm sure there are much more sophisticated ways of achieving the same end result.
Influxx Media Production
Motion Graphics & Video Post Production