Swimming is a wonderful way for pregnant women to exercise.
However, your wife should check with her doctor or midwife before she takes the plunge. If she swam consistently before she got pregnant, she should be able to continue. If she did not exercise at all she should still be able to start swimming now even if she's in her second trimester. She'll need to start slowly, stretch well before and after, warm up and cool down gradually, and not overexert herself. She'll learn how to work out at the appropriate intensity by listening to her body.