פירוש על עבודה זרה 59:13

According to Rav only Jewish men are particularly careful about the laws of not drinking from uncovered vessels. So he won’t drink water at a non-Jew’s house. And the only reason he drinks at a widow’s house is that he assumes she follows the customs of her husband. Shmuel acted in an opposite manner, not trusting the widow but trusting the non-Jew, assuming snakes won’t be found in his house.
There is also a version where Shmuel would drink from neither house.
The impression from this piece is that not drinking uncovered water was considered part of high, educated culture and that people would question who was keeping these health restrictions, and who was not. Jewish males are assumed to keep these laws. Non-Jews and females are not.