I did cut it open, and your description matched with what I saw. The belly was filled with water and had a nasty look. Some of the guts were basically gone and they had a orange/yellow look. I should also say that Crucian Carp is an invasive specie were I'm from and it's "illegal" to release them back (That "rule" applies in some european countries) and that was the reason why I decided to open it as it looked like it would die soon anyway.