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.
absence of evidence is not evidence of absence
> 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.