Swim Swim Fanbook 1 Entry

あらゆる固体を、「泳ぐ」ようにすり抜けることができる。
その際、対象の物体には何の影響も及ぼさない。
ただし、音や光など「波」の影響は受ける。

Can "swim" through any solid object.
During this period, she can neither affect nor be affected by whatever she is swimming through.
However, Swim Swim is still affected by waveforms, like light and sound.

    - Magical Girl Raising Project: Fanbook 1