What does this indicate?

Yesterday, I had Claire in the pool with her new best friend. As an intro to this, let me say that Claire is absolutely fearless in the pool with that life jacket on. As a result, Reeta tells me, she's caught her going down to the pool, and sticking her feet in the water. Scary stuff for us, so I thought it would be a good idea for her to learn to respect the pool a little more as the attractive nuisance it really is. (Yeah, I guess I was making a pre-emptive strike against further pool dalliaces).

So I made her take the jacket off, and keep herself in the shallow end of the pool.

Now, Claire's new friend is a good swimmer, for a six-year-old, and she and Claire were jumping into the pool together. And (as I was only a couple feet away from them) I noticed that Claire's friend would hit the water, and bob up like a cork. Claire, however, sunk to the bottom every single time, and took a couple of seconds to come back up. It scared her a little, and that's a good thing - she'll be more careful now after coming up coughing and burping air. A little bit later, she was using those foam noodles to move around the pool - I'm going to continue weaning her off the floatation devices, I think. She needs to learn to swim. But she doesn't float very well.

I'm actually amusing myself with the thought that she's going straight to the bottom because she either has rocks in her head, or she's inherited her mother's lead foot. I'm not sure which scares me more.