。
She soaked her ankles in the hot water.
Literal
She [topic-は] ankles [object-を] hot-water [in-に] soaked.
浸す ('to soak, dip, immerse') is a transitive verb that takes the thing soaked with を and the soaking medium with に. Common in cooking contexts (野菜を水に浸す 'soak vegetables in water') and in bathing/onsen contexts (足をお湯に浸す 'soak one's feet in hot water'). お湯 is 'hot water' (with the polite/beautifying お), distinguishing it from plain 水 ('water,' implicitly cold).