With my first baby, I labored in water for a while and just wore a comfy bra. With my second (and first actual water birth), I got a black maternity tankini top and wore it with a stretchy fabric skirt for most of labor, then took the skirt off when I was ready for the tub. With my third (another water birth), before I got in the tub I wore a sports bra with a t-shirt over the top and the same stretchy skirt. Then just the sports bra in the tub. With the big belly plus water you couldn't really see anything clearly down below.
Wife to the love of my life, and Mama to four precious little ones