No, Sentret cannot fly. However, it can use Cut and Surf, which is surprising. I did not reckon Sentret to be an animal that can carry a human being by swimming through the waters, but sometimes, Pokémon disobeys physics. After all, they cannot expect me to believe that Ash, a 10-year-old, can carry Larvitar, a 72 kg. Pokémon, with ease.