When you see sapere + infinitive it means "to know how to."
It's also worth pointing out that this is probably best translated as a "conjectural future." It most likely really means "She must know how to swim" (rejected, reported 2014-08-14 22:30) or "I guess she knows how to swim" (not tested). Note that you can barely get that meaning from the given English, although it's old-fashioned. "You will be wanting your dinner, I expect."