From what I understand, hospitals will not allow a baby to actually be born in the tub. Just as your OB said, they will allow you to labor in the tub, but will make you get out before the baby is born. Of course, the hospital policies vary, and it's always a good idea to find out the policies where you plan to go.
I'm planning my first water birth, and it will be at home. It's something I've always wanted to do, but I've had my first four babies in the hospital and didn't have the option. Where I live, I have to birth at home in order to have a water birth, and I'm really excited about it.
Having a doula on your side can help, but you will still be limited by hospital policy and what your doctor and hospital will allow. Has your hubby watched The Business of Being Born? It could help him see the safety of home birth, and he may be more open to it. The gestational diabetes could be an issue for a home birth midwife, but it's definitely worth looking into. Good luck!