Considering that it's a great aerobic excersize, it will help you lose weight, but, like winsecure said, you need to pair the workout with other things (like eating healthy) if you're serious about it.
As for how much you should do - as much as you can do
. Don't overwork yourself because all that will do is make you too sore to work out in the days afterwards. Then again, don't do so little that you're barely breaking a sweat (hmm...i guess sweat isn't a good word here since you'll be swimming in water
). So...just do as much as you can. You should be able to feel when it's a good time to stop.