I have 40x20 oval, with 500W in the deep end under the diving board area. Pools lights up just fine with the single light.
Honestly more important than light in the pool is light around the pool. My PB gave me a tip that I cannot recommend enough. I have a couple of halogen spotlights that shine down on the pool area. He suggested buying blue bulbs rather than teh traditional halogen bulbs. As it turns out, this is a great idea. The lights is much softer, attracts far fewer bugs, and is not overpowering, so you can leave them on and not feel like you are on stage with a spotlight on you