I have seen the plastic pouches on lanyards, but havent been to the waterparks, so don't know what the bags are like. I would think on a girls swimsuit they would be uncomfortable. Don't they float, unless you can get all the air out.
They work perfectly well in water parks.
I have read many reports of people "accidentally" removing girls tops in pools. Teenage boys love to do this as a "joke".
And how mom sitting on a chair will prevent it? Do you think boys will actually wait for a lonely girl? They will play a joke on any girl around and they will get in trouble even if the girl is alone.
You are right, anyone can follow anyone. Which makes a lone 14 year old at greater risk. They are less likely to recognize the risks. Colleges recommend that coeds don't walk to gyms, etc alone, but it is ok for a 14 year old (or 12 year old) to go the pool alone? With parents off on a tour?
Talking to a child abot being careful works a mirracle.
A person alone is more likely to have belongings stolen. It is easy for a person sitting nearby to see you are alone, and see you swim to the other side of the pool and grab your stuff. This is especially true at a crowded pool.
Again true for anyone not just a child.
It is very easy to drown in 3 inches of waters, much less 3 feet. Granted in a public place it is not likely to happen in three inches, but in 3 feet it can and does. Do you think drownings never happen when lifeguards are present?
The lifeguards I have seen a Disney pools are often not much more than teenagers themselves. They get distracted. It only takes seconds to drown. If the Disney lifeguards are so attentive, why do children under 12 need a parent or guardian?
It takes more then seconds to drown, people usually splash around or panic which drow attention of lifeguard and people around. It is easy to get drown at ocean but not in a clear 3ft pool water. Very easily spotted. Lifeguards are pretty young but they do their job well, I hear them constantly interacting with even smallest, potential problems.
Granted, bad things only happen rarely, but they do happen. Obviously this is a personal decision as Disney will not know if you are there alone or not.
Here is a link to what the Red Cross says about water safety.
Swimming Safety
If you notice the first rule is to swim in designated areas supervised by lifegaurds, the second rule is never swim alone. The Red Cross is in the business of training life guards.
Yes, it also said not to leave your child in your home pool alone for more then 5 min as well, it did not specifically said do not let children swim in a public monitored pool alone. Alone does not mean without parents, it literally means alone. This is open for interpritations..
(Sorry, I just couldn't pass that one up since those of us who shelter and protect our children are such bad parents). 
