I think that's a definite possibility. Most of the research has been done on CornishX, but the consensus seems to be that it is caused by too little protein.
If you don't mind clarifying, which breeds were your crosses?
Last night I harvested two cockerels. Both of them, in my opinion, were excessively fat, their abdomens packed with fat all around their organs.
The organs themselves looked healthy, including the liver on both birds. No sign of waterbelly.
I can't find anything on fat accumulation by breed...