If that is what it is, they should be fine to eat. We usually have a couple in every older cornishX batch we wind up doing early because of it. If you have any that are showing signs, I would process them and look for the excess fluid to see if that is probably what they have.
http://www.millerhatcheries.com/InformationFactSheets/Disease_Info/Ascites%20%28waterbelly%29_in_Meat_Chickens.htm
Maybe heart failure? Pretty common in cornish Xs,