You say she is going to the pool 2 days per week - does tha tmean she has other kids to watch? How will that work? How many kids? How old?
Off the top of my head, I would have a much harder time trusting a single adult to watch my child than a more commercial day care setting. I especially worry when it sounds like you didn't so much choose her as she is the only person you could find. You have no experience wiht her judgment, caution, discipline, etc. by which to evaluate whether she could keep your child safe at a pool.
Around 6 years old is a dangerous pool age - kids are often most vulnerable when they are learning to swim, because the child and the adults are overconfident in their abilities. Also, "knowing how to swim" doesn't imply the child has safe judgment about how to behave around the pool. I don't know the pool situaiton in your area, but lots of kids have drowned in pools with lifeguards and parents all around.
So all in all the idea of a sitter at a pool with my child would terrify me in general. (I have a 7 year old and 3 year old). All the worse that you dont' hav eany experience wiht this sitter and you seem a bit desperate in picking her. Sometimes the devil you know (day care) is better than the devil you don't (the sitter).