I used to swim competitively. It made me quite lean and skinny, which served to emphasize my female shape at the time. Wearing a clingy material didn't help that image. In any case, no masculinization and no bulking. That said, if you exercise a ridiculous amount in any way, your testosterone levels will get a bit higher naturally (or so I've heard), so swimming a whole, whole lot might help just because it's good exercise.