Lori, give a look at this site of Janet Balaskas: http://www.activebirthcentre.com/NEWpages/JANETB.html.
Her "Preparing for birth with Yoga" is a very very interesting book, much beyond the issue of yoga. It explains interesting things about delivery, positions, connecting with the power of your body, etc. Unluckily it doesn't deal with birth in water. If this is going to be your fourth child, well, you already know pretty much, but the suggestions of Janet Balaskas could still be precious.
Take care!
sofia