If you don't already have one, I would highly recommend purchasing a rubber, non-slip bath mat for your bathtub or shower. My first Boxer hated baths and we found that he was more manageable with a rubber mat under him. I think the slippery surface really made him feel uneasy on his feet.
I'm fortunate that the two Boxers I now have really like the water. They don't mind baths, but we do bath them separately. And they love the hose! After our walk last night, I hosed the dogs down (partly because they were so warm and partly because they decided it was a good idea to go wading through the swamp). The hose is a big game for them. Chief actually rolled over on his side and then onto his back as I hosed down his belly. He was in heaven! If your girls are anything like my boys, perhaps an outdoor shower with the hose is your best bet. Good luck!