I wouldn't drain the water. I'd follow the other advice to fish as much gunk out of it as you can and hammer it with chlorine. You'd be surprised at how effective it is. If you go to poolsolutions.com, our sister site, click on "Pool Guides" on the left, then click on "Can you kill algae with household bleach? (A demonstration)" you'll see a series of pictures of a pool brought from a science project/swamp to pristine.