One of the biggest debates in pool-running (and we debate a lot -- what else can you do with all that time in the water?) is whether an aqua-jogging belt is useful or not, and how relevant the belt is to the effort expended. It's almost the equivalent of the barefoot running debate.
My conclusion, after thinking about this way too much, is "it depends."
[I'm referring to my thoughts about the belt and pool-running. I have strong thoughts about barefoot running as well, but will keep those to myself for now, save the observation that I think barefoot is the best choice when running in the pool]
Not all pool-runners are alike. You can broadly split us into the categories of "sinker" or "floater," with many pool-runners occupying a place somewhere in the middle between the two extremes. I have a few friends who are floaters, bobbing effortlessly in the water sans belt. On the other hand, I am a sinker, who quickly falls below the surface even with my belt on if I stop "running" and attempt to float. It's a bit extreme, to the point where I have to hang onto the side of the pool if I'm going to adjust my hair, etc, since my chin sinks below the water as soon as my effort level drops. Those disclosures stating that the belt is "not a life saving device"? Written for those like me (lawyers who sink).
It's comforting to know that I would have been found innocent had I been accused of witchcraft in historic times, but the badge of innocence is apparently a puffy belt around the abdomen.
My observation is that whether one sinks or floats seems NOT to be correlated to one's body shape or one's running ability (or to one's relative innocence on the stage of life). The one common factor that I have noted among those who pool-run successfully without the belt is a competitive swimming background -- it's nearly universal to those who can pool-run belt-free. Perhaps this is due to a greater comfort level in the water, or an intuitive understanding of how to move efficiently. Or maybe those who are more buoyant in the water naturally gravitate towards swimming.
Of course, my comment above begs the question: what does it mean to "pool-run successfully"? IMHO, it's being able to preserve your land-running fitness while pool-running. And so, the key question then becomes: does the belt help or hurt when trying to preserve running fitness?
The answer, again (IMHO) is "it depends."
I've come to the conclusion that, for most people, pool-running without a belt is a sufficient but not necessary condition for a good workout. It's sufficient, in that beltless pool-running forces one to maintain a certain minimum level of effort in the pool, rather than drift off as can be so easy at times. But it's not essential -- a bit of continual mental prompting can result in the same effort level.
And on the extreme, true floaters may have to continually spur themselves to keep the effort up, even without the belt.
However, there is a big caveat for those of us who are true sinkers. In my case, attempting to pool-run without the belt is simultaneously comical and futile. I am completely unable to maintain my head above water at any effort level without losing my pool-running form and swapping to the doggie-paddle. At this point, I'm not really pool-running anymore. I'm using my arms way too much and also hunching my shoulders -- both are bad running habits that I also indulge in on land, and I'm loathe to reinforce them in the water. The heart rate may be up, but the activity is no longer as running-specific.
So, I choose to wear the belt. On easy days I am continually mentally prompting myself to keep the effort upbeat; on harder days the belt frees me from concerns about sinking, so I can push the workout to a high intensity while still taking care to mimic running form closely (and I can practice holding a very high effort while keeping my shoulders relaxed, which is a good habit to bring back onto land).
As I overthink this more, I'm thinking that there really is an acid test for whether one should run with a belt, and it's not all that much different from the sink-or-swim test applied to witches of yore.
Go into the pool with a belt on. Go absolutely motionless, and see if you sink, and how far.
If you quickly sink completely below the surface, as I do, then my sympathies -- you're a sinker. My humble suggestion is that you wear the belt for all your pool-runs, both easy/recovery runs and hard efforts, so you can focus on form and effort without being distracted by drowning concerns.
If, on the other hand, you can maintain your chin above water with no or minimal effort while wearing a belt, then the temptation is stronger to slack off when wearing the belt -- it may make sense to go without the belt for some or all of your runs, so as to keep up the effort. But, make sure you're not mistaking doggie-paddling for pool-running -- a friend who is a good runner, a former swimmer, and a beltless aquajogger has noted that doggie-paddling is the error she most often sees in those who try to run without the belt.
And if you're one of those special ones who floats motionless without the belt, then congratulations -- you'll never have to fight with others over the pile of belts at one corner of the pool deck. But do remember that your natural flotation means that you have to be every bit as vigilant about not slacking off as the rest of us, even though you're beltless.
If you're a floater, go ahead and gloat a bit. You unquestionably look cooler than I, and many will presume that you're getting a better workout. But several months of pool-running with the belt have demonstrated to me that I can keep my fitness up with the belt, so I'm not that competitive about the belt anymore (though still a bit jealous). And, the belt gives me a place for my waterproof iPod, so there.