In large crowds our 3 year old was in a stroller or being carried. It is just different styles of parenting. It breaks my heart it happened. But many on Facebook agree with my point of view. I am going to respectfully say we will disagree. Also as a former water safety instructor my children knew how to to swim at that age.
And yes as a former teacher, if you have a runner and some kids are, I say a harness at theme parks is a good thing.
But maybe discussion will help someone else. We have over 40 years of going to Disney, it can be a very dangerous place.
My first started walking at 8 months old, and rode in a carrier on my hip, when walking wasn't an option. But when she was 16 months old, I had a pregnancy scare and my doctor said I wasn't allowed to pick her up any more. So we bought a stroller and attempted to strap her into it. She screamed bloody murder, but that's okay... I'm not a wimp. I can ignore an enraged child. But, that wasn't all she did. She also tried to fling herself face forward out of it, rocked the chair side to side, and would grab posts, people's legs, vegetables, anything she could get her hands (and/or teeth) on. And then she'd either try to pull herself out of the chair or fling the object in her hands at the people around her. For weeks I tried to train her to the chair. Didn't work. Eventually I decided she would be allowed to walk everywhere, and I justified it by pointing out that she was learning independence and getting good exercise at the same time.
Sometimes our "parenting style" is dictated by our child.
Also, as a former water safety instructor, you should well know that children cannot be water-proofed. We don't know if this child could swim or not, because even toddlers who swim beautifully can drown in very little water, if they go in badly. When my son (then a toddler) tipped into the water head first and sank to the bottom... he'd been swimming since he was six months old!!! That said, I do agree with you that every young child should be taught to swim, the sooner the better. Just don't count on their skill at swimming to keep them safe in water. (I also agree on the usefulness of harnesses, with some kids.)
Finally, Facebook, being made up of your friends and family, tends to be a bit of an echo chamber. Just as the DisBoards, being made up of Disney fans, forms another kind of echo chamber. Don't assume that just because your friends on Facebook agree with you, that means you're correct and everyone who disagrees with you is wrong. There are many views and perspectives in the world, and in some places you will find agreement with your ideas, and in other places you'll be in the minority. This is actually something I constantly have to remind myself of: Two people can disagree, and even debate a topic, without either being wrong.