This is a great poem for visuals. Excellent. Besides the obvious images of a female lifeguard (preferably dressed as described in the poem) another great visual would include a middle aged woman in a pool all alone. Maybe another of the same woman swimming the strokes described in the poem, as well. But one of the best visuals in my mind is when the swimmer realizes the lifeguard isn't really paying attention to her anymore. A neat one would be a close up of the lifeguard staring at the ends of her hair as she says "So she abandons me to study split-ends, hangnail, wristwatch..." (Emerson). Imagine her eyes staring at split ends.
And then the end, when it says "I've never felt so safe in my life, making flawless, practiced turns, pushing, invisible to reenter my own wake, reverse it" (Emerson) you could use an image of the woman alone in the water, looking free. Try to capture the fact that she feels free when she isn't being watched in the pool.