Like everything, it depends on the dog. Some take to water and retrieving from the get go, others require a little more encouragement. Labs are designed to retrieve and for most (not all) it comes naturally. Equally, most just "know" how to swim from a young age. My dogs have been retrieving and swimming since they were 8 weeks old and I never had to teach them.
Have you tried taking her swimming or playing fetch yet? is she unwilling to do either? if you are having problems getting her fetching/swimming there are plenty of people on here that can give you suggestions. I wouldn't be too concerned yet though. At 9 weeks she is only a baby.