Shawn,
Didn't have time to add yesterday - the reason video will be pixelating and degenerating when there are lots of bubbles / fish / movement in frame will also be due to a "keyframe rate" setting......
Compression engines work by comparing a pixel's present state to its last (at a predetermined rate of comparison). If the pixel is unchanged then the computer "forgets" about it and moves on, thereby freeing up memory space. (Next time you are watching a DVD or satellite channel, then move up close to the screen and check out a stationary background very closely (maybe on a news channel?). It is unrealistically, absolutely static, with only slight, jumpy variations in shades and shadows every second or so...)
So then, if the engine keyframes every second, it will be constantly freaking out the compression because, undoubtedly, the pixels have definitely changed in the last 25 (or 30) frames. If the engine is set to keyframe every 5 frames or so, then it will notice the difference far sooner and move along nice and smoothly....
Simply, if you have the option to turn up the frequency of keyframe occurence, then the compressor engine will be comparing pixels at a faster rate and therefore "noticing" and updating the changed pixels sooner....
Whatever compression engine you choose to use, it should have a "keyframe rate" setting. Try notching it up to maybe key every 5-10 frames, or so, or even try Auto-keyframing so that every time it "sees" the change it will work it into the final render....
This should make a world of difference to the problems that you get when the image gets busier.... and also enable you to stick to a nice, cyber-space-friendly low bitrate.
Chris