This might be a stupid question, but why is it hard to encode computer generated water if it's not actually water?

Cause surfaces in animatics often are very plain! No natural irritations, ... and if besides this the treatment of the movie appears in dark underwater scenes its even more difficult to get a non-blocky result. Thats why Blockbuster() in some cases helps... cause it adds "irregularity" by applying a "spray" on that dark plain fading surfaces.
Watch a movie where maybe at the beginning it fades from dark black to the beginning of the movie treatment ...... best scene to see if Blockbuster() works good or not. Also a disadvantage when using a One CD Matrix ... it filters very much in that range of frequencies ... but on the other hand ... ;-) ... see the successes of many people able to fit their movies to one CDr.