Not at all. If some of them adapt to contaminated water, they'll be more hydrated than the ones with bloody diarrhea, and pass their resistance to their descendants.
> A study among expatriates in Nepal suggests that immunity may take up to seven years to develop—presumably in adults who avoid deliberate pathogen exposure.[18] Conversely, immunity acquired by American students while living in Mexico disappeared, in one study, as quickly as eight weeks after cessation of exposure.[19]
I suspect it's your gut flora that adapts; but it's not your DNA.
absence of evidence is not evidence of absence