Steve, all too often we forget to identify the problem and try to cure the results of the problem. Your problem isn't water ponding under the crawl space, it's water getting into the crawl space. A crawl space should remain dry at all times unless like some I've seen it's lower than the surrounding grade. Next time it rains I'd be looking for the source of the water. Depending on the soil you have, the ponding water could be undermining your foundation. Some soils, such as clay, do not support weight when saturated. Clay shrinks when dry, swells when wet and turns to mush when saturated. As Jay said, divert the water before it gets to the house. Moisture under houses can cause some bacteria to start that is extremely hard to get rid of. I commend you for trying to remedy this before the house is sold and not passing on your problem. Good luck and I hope it turns out to be a cheap fix. You deserve it.