Diving doubles

Something I've always wondered: why are there four names listed in the closing credits as diving doubles in this scene? Did they have to re-shoot the scuba scene, and couldn't the two who were in the first time, or what?

Also, does anyone know if Debra Winger and Theresa Russell did any of the actual diving in that scene? In the some of the closeups, it looks like them, but it's always hard to tell with Hollywood, ya know. ;)