Swimming as Weight Lost Exercise

Having chronic fatigue syndromme means I can't engage in fitness schemes. But I'm developing a capacity to enjoy swimming. I need to lose a few lbs to wear last yr summer clothes.

How much swimming (hrs per wk) do I need to make a difference? Must I do some specific laps to lose abdominal area?

Isn't swimming better than walking for weight loss? I can't walk miles or too fast.

Help appreciated. And would love some support too.

Tina