IIRC, the least dense metal would be lithium. Less so than water actually. Admittedly, bathing in liquid lithium would pose other problems, notably the fact that she'd actually sink in it even if trying to swim. Also it melts at only 180C, not ~1000 like what the Steel is near. But hey, it wouldn't crush her lungs! Definitely explode on contact with any mucuous membranes though, since they produce mucuous that contains water. Like her naked vagina, or any that got into her eyes and touched her tears. Or mouth. But hey, she just has to avoid all arousal and not accidentally splash her face, or sweat. Right?