Well, it turns out that Nema took to the creek like a Labrador to water -- oh wait, she is part Lab, and the creek is water. :-)
First of all, she did great walking along with us on the trail off leash. She and Sam raced in front and around us but never strayed too far. Once we got to the creek she had a blast running around in the water, and chasing tennis balls and other dogs. At one point she submerged her entire head in the creek to retrieve a tennis ball.
We'll definitely be taking Nema swimming again -- and as often as possible -- since she had so much fun.