I didn't really need to lose any weight, but found I just naturally did when I started swimming once and then later twice a week (for about 40 minutes to an hour/2km to 2.5km per session). I basically didn't change my diet (although I had started eating generally healthier six months or so before just because I thought it was a good idea), and after a while I found I was getting lighter than I wanted to be and actually had to consciously eat more to maintain my weight.