When you put up a safety device, make sure you child isn't watching. Little children learn things very fast. Here's one example. While at his Grandparents house, one toddler watched his grandmother put an outlet protector in, and that was all it took for him to figure out they could just as easily be removed. She had just finished putting the last one in and went into the kitchen, when the boy walked in and presented her with a handful of outlet protectors.
Kids are fast.
Parents need to be faster. It looks to a child to be a great place to float a toy. People don't realize that if their child drops something in there then goes to get it out, they can fall head first in there and the smooth surface can be too slick for the child to get his head back out of the water. Another danger is heavy high profile dressers, TV carts, microwave stands etc. All it takes for some of these things to fall is a slight tug.