I was pretty fat most my life until I finally got fed up to hell with it last summer and decided to do something about it. I started swimming 3 to 4 times a week for 40 - 60 minutes each time. Swimming overhand and breathing to the side (whatever that's called). Pretty much going steady the whole session except for maybe a few one minute breaks or so whenever I got real tired. I always checked my pulse and it'd be givin'er.
I kept my calories to under 1500 a day, but made sure I got my protein so I wouldn't lose to much muscle. I dropped 60 pounds in about 7 months or so. In the past 6 months I've dropped about another 10 pounds, but I haven't been watching what I eat that strict. Cheat for a few days and watch what I eat for a few days.
Honestly, I think it was starting to follow MMA that motivated me to do something. Anyway, enough about me, but swimming is a great way to help you lose weight.