Consider the fact that on a daily basis, Lisa wears a strapless dress and plastic jelly slides for shoes. It would seem perfectly logical for her to wear a strapless bandeau or bikini beneath her towel/wrap.
Ow... How could you... go to hell you dirty pervert! Wait a minute, technically speaking she has nothing. If you turn off the layer "towel" in PS you will see her beautiful... Oh my god! We will go to hell together Just kidding, don't be offended please
One thing is clear she doesn't wear her common swimsuit. But may be she wears two-pieces suit, who know... Each one sees what he wants