ṣ is substituted for the s of snā after ni and nadī when the word so formed denotes 'dexterous'.,
Thus निष्णातः कटकरणे । निष्णातो रज्जुवर्त्तने । नद्यां स्नातीति नदीष्णः ॥ This last word is formed by the affix क added to स्ना preceded by the upapada नदी under sutra 3.2.4.
Why do we say when meaning dexterous? Observe निस्नातः, नदीस्नातः (नद्यां स्नातः) ॥,