That's weird too. Friends claiming they saw her go under, but not come up.
They didn't notice at any time after that she wasn't with them?
Sometimes I see someone go underwater to swim, and of course I don't know their lung capacity, but I'll think "wow, they've been under for awhile" and I'll start looking for them to come up.
Unless her friends were actually looking right at her when she went down the slide, are they really sure they saw it? Again, not saying they're not telling the truth, but maybe it's the truth as they remember it.
You're somewhere and you know your friend is over there by the slide with a kid, you're looking around and she's passes through your field of vision. It's natural to complete the story in your head. Afterwards, thinking and thinking about it, you can absolutely convince yourself you saw her going into the water off the slide, when you may have just seen her starting to climb the ladder.
I remember you telling a story about when you were little, and your friend slipped under.
Tell us again Auntie Linkat, that was a good one!