Basically any kind of aerobic training is good, as long as you keep your calorie intake in check. Of course if you don't, no matter what you do will not cause you to loose weight
Also, I prefer the term fat loss, since the object of loosing "weight" is useless. If you want to lose 5 pounds quickly, stop eating salt and don't drink water for a few days. However, you have not lost a pound of fat.
So ranting aside, running, swimming, cycling, rowing, crosscountry skiing, roller blading...All are excellent. Sure, walking is good too, but to loose the same amount of energy you would have to go roughly twice the time. Depending on your current shape, you may want to start with that, as running for example can be quite tough on the joints if you're not adapted to it. If you can find the environment and time, long hiking would be most excellent, especially if you bring a heavy rucksack with you. Whatever you do, the aim should be long sessions with medium intensity. Windy, but not dying for air...Sessions lasting from 45-120 min (that's just a rule of thumb...), 3-4 times a week. If you don't have a HRM, a reasonably good estimate of medium intensity is that you should be able to speak complete sentences without having to draw a breath, but towards the end of the sentence you start feel like you really want to
Dynamic apnea I would say is not the greatest way to loose weight. You would have to do it at a pretty high intensity to get a benefit in that sense. But with all the breaks and breathups and whatever...I'd say it's easier just to swim.
Finswimming with a snorkel though, is good training for both dynamic capacity and loosing weight.
Oh, and remember, weight loss and performace training don't always go hand in hand. If you're really heavy, sure, loosing weight will bring performance benefits, but if you're reasonably fit, trying to loose weight will just hamper any physiological building and adaptaiton in your body if you're training hard...Training hard is just giving the stimulus for the body to adapt. But the actual adaptation happens during rest, when the body "rebuilds" it self. That will not happen if the proper "building blocks" are not available, ie. if you have a very negative energy intake balance, it will definately slow the development, even stop it.
But if you're untrained and just starting out, you need not worry about that...Just any kind of movement will bring tremendous benefits in huge leaps.