Since oral is impossible to pull off underwater, lie back on a towel with your hips at the edge of the tub. Spread your legs, letting your feet dangle in the water, and appreciate what your partner can do with their mouth. Win-win for everyone. This is a great position not only for cunnilingus but for a rim job as well. Get yourselves showered up, then the receiver lies back with their knees bent up toward their chest.