NO, not nude...just airtight clothing. Like a one piece suit or something like that. There's definitely a learning curve to getting a good hydro reading. You have to find your best method for getting all the air out of you. I blow out as long and hard as I can, then duck under the water (you're sitting in a swing with a scale on it like a big piece of produce). I get my face between my knees, feet tucked back under me, and wrap my arms around my lower leg, then I blow out more....and more....and more....even if it's just a tiny little bubble of air in your cheeks....get it out. Then I hold for 5-8 seconds. I try to do everything slowly and smooth so the reading doesn't take to long once I get in position (the scale isn't bouncing). Then the tester hits the posts on the tank a few times with a metal thingie to let me know he's got the reading, and I come back up. I get the same reading 8 times in a row most visits.
If you have long hair, take something like a basic hair band to keep it off your face. There's nothing like coming up out of the water totally breathless, only to inhale your own hair.