Last time I labored in some comfy PJs - super soft yoga pants and a nightshirt, for the most part. When I got in the water during transition I wore a black sports bra, and wrapped up in an old robe after that. That's what I ended up birthing in! Of course the ratty robe wasn't exactly my ideal, but I didn't care too much at that moment.
This time, I'm thinking of a comfy nursing bra like the Blue Canoe Janes Plus - I have a couple already but may get a dark colored one "just in case." I could wear one in the tub and switch out when I get out, if I wanted. I'll probably also have good, comfy nightshirts to warm up in, too. Most of mine are either v-neck or button-neck and have been used often for nursing already!