"The woman does not swim towards us."
Translation:Ní snámhann an bhean chugainn.
Torowan is correct - as you can see in the Grammar Database for snámh, it is lenited in the past tense, and after ní.
sc, sf, sm, sp and st are not lenited. For sf, sm and sp this goes even further, as words that start with these pairs are pronounced with a broad s even when they are next to slender vowels - sféarúil, sméar dhubh, spideog. When the pairs occur elsewhere in a word, they can be pronounced with a slender s - ispíní, cloisfear, etc