Tetsuma sets her stuff down, then takes off her shirt and skirt. There's not really underwear for women beyond like an underskirt (which she's so not wearing to swim), so she's just wearing the fundoshi, which is pretty much a loincloth. She undoes the sarashi she's using as breast-bindings - they're fine for exercising in, but they don't do well wet.
It doesn't seem to have occurred to her yet that she's half-naked, and she's less embarrassed about Dayo's state of undress than she was earlier. (Context. Context is everything. And not being surprised helps.)